Single Transistor Voltage Controlled amplifier! (VCA)

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/ theaudiophool
This time we're going to use only 1 transistor (BJT) to make ourselves a pretty decent sounding (if you ask me!) VCA. On the way we'll learn the very basics about how transistor circuits work, and we'll learn how to turn currents into voltages using op-amps!
Schematic: drive.google.com/file/d/1y8wO...

[@StephenBrown85]

This is a nice simple circuit without too many confusing components. I think your description of a transistor as a voltage controlled amplifier is not quite right though - really it's a current controlled device. The base emitter voltage is always 0.6v so it can't really be voltage controlled! I've never seen the op amp current to voltage configuration and I didn't understand your explanation of how it works. Also I found the pen lid clicking on and off a bit distracting. I think this video is a great introduction to transistor circuits. It's probably not the cleanest VCA ever but there are a lot of times when you don't need it to be perfect (and sometimes a bit of character is desirable). Keep up the good work - I'll be back to see what else you come up with!

[@TheAudioPhool]

Hi Stephen! This is quite a common misconception about transistors, i've had this comment a few times so i'm going to answer yours and pin it - i hope you don't mind! Is a transistor voltage or current controlled? Really you could argue it either way, but to describe a transistor as voltage controlled is quite accurate. What is really giong on inside a transistor is that (just like in a diode) there are electric fields inside the device that counteract the applied voltage (input voltage) in order to dictate the flow of current in the device. These fields can grow and shrink depending on a number of factors but most importantly in this context, the applied voltage across them. By then biasing the 3 different terminals to different amounts (usually Collector > Base > Emitter), these fields then restrict the flow of charge differently, and we can control the flow of charge through the device through the modulation of these applied voltages. There's a quite well known equation that describes this behaviour relatively simply called the 'ebers-moll equation' Ic = Is(e^(Vbe/Vt)-1) IC is the collector current, Is is the saturation current (this is essentially a constant at room temp), Vbe is the base-emitter voltage and Vt is the 'thermal voltage (again we can consider this a constant at room temp.) This equation shows that the collector current as an exponentially varying function of the base to emitter voltage - not current. The current that flows through the base is developed by this voltage. I think the confusion comes at the approximation that the base to emitter voltage remains constant at 0.6 volts. This is only an approximation.The reason we make this approximation is because we are designing our circuits so that the exact value of Vbe isn't critical, not because it is actually fixed! Vbe can vary from device to device and varies with things like temperature, so we can't rely on it being any particular value. So in some contexts it's easier to 'pretend' that Vbe is fixed and that the input current varies, but this isn't what is really going on. It is small fluctuations of this Vbe that cause the changes in collector current (see: i.stack.imgur.com/P6UOD.png or www.onsemi.com/pdf/datasheet/2n3903-d.pdf figure 17 here on page 6 or any transistor datasheet!), and the change in the base current is actually due to the small change in Vbe. If we could get a device that could vary the input current with no change in input voltage at all, we would have a device with 0 input impedance which can't exist! I think the saying "all models are wrong, some models are useful" is pertinent here! Modelling a transistor as a current controlled device that amplifies the current from the base at the collector is a very simple and easy to use model, so we use it often! but that isn't the whole picture. Even using our exponential model above, which is more accurate, is still wrong! it doesn't account for the early effect and other things. But it's useful, so we use it. :) So while it's not wrong to say transistors are current controlled if that's how you like to think of them, it's also perfectly correct to think of them as voltage controlled as well. Really it's both! Voltage and current are intwined, you can't have one without the other. Ultimately it depends how much detail you care about in the context of what you are trying to do which model you use, and each model is 'wrong' in some way or another. Usually we try to go for the simplest model that gets the job done. I hope this isn't overly technical. Let me know if you want me to expand on anything here. :) I have a video on the physics of how diodes work which should shed some light, and i'm planning on continuing that style of video through all the different transistors. The book 'The Art of Electronics' is a relatively beginner friendly book and does a great job explaining transistors in great detail. Chapter 2 starts with the 'current amplifier' model and takes you all the way up to ebers-moll and beyond, I highly recommend it! Thanks for the great question and thanks for watching!!

[@TheAudioPhool]

Sorry you didn't understand the Current to Voltage converter. To try and simplify my explanation, The input of the op-amp has infinitely high impedance (approximately) aka it looks like a hige resistor. Because of this, the current from the output has nowhere to flow but through the feedback network, because electricity will always take the path of least impedance. This current develops a voltage across that resistor. This drop is positive at the input and negative at the output, by convention. In order for the op-amp to have both it's inputs at ground (op-amps want both their inputs to look the same) the op-amp lowers it's output to accomodate the voltage drop caused by the input current. i.e if the input current cause a 2v drop across the resistor, the output moves down by 2v so that the input is at 0v, and the output is 2v lowere than the input, because we must have that voltage drop across the resistor! tinyurl.com/yg6ctfnt There's a simulation that hopefully helps if you're a visual person.

[@StephenBrown85]

@@TheAudioPhool Great reply! "All models are wrong, some models are useful" is a great quote. Thanks for the detailed explanation!

[@StephenBrown85]

@@TheAudioPhool Thanks for clarifying - very clear and understandable explanation! The key thing is that no current will flow into the input. I always find it hard to conceptualise how current can flow into the output of an op-amp. I know that it can, but it feels really un-intuitive. Thanks for replying to my comment - I've learnt a couple of cool new things! Much appreciated!

[@TheAudioPhool]

@@StephenBrown85 That's the only problem with the 'black box' op amp approach. It hides so much functionality inside a mysterious box that some behaviour seems really un-intuitive. I'm glad i culd clear that up at least. Electronics is weird and confusing!!

[@TommyBoy7Heads]

Found you via reddit. I'm 5 minutes in and already this is so informative.

[@svofski]

This circuit is used for ADSR envelopes in a Soviet synthesizer from the 80s called Maestro. It has 4 channels, so the currents from all 4 channels are added together and converted to voltage using just one opamp. I didn't understand how it works before your excellent explanation. You're a great teacher. Thanks!

[@TheAudioPhool]

I never knew that! I thought i made this up myself!!!!

[@woosix7735]

Hello there. Do you have the thematic for this Maestro somewhere? I would love to take a look but I could find anything on the web

[@kevinscollan8293]

The way you explain things and go step by step; I learned something of transistors years ago but I think I actually understand the base-emitter relationship now. WHOOP !!!

[@densebutton]

This is great stuff, just what I've been looking for

[@stibbons5924]

The lord's work right here

[@joeyhendriks6216]

Your videos are really Golden!! Combines theory with practice

[@heeeyno]

beautifully explained. it may just be because I've studied this just long enough to have a lot of things click just by watching this video, but I think your added details and asides explained exactly what I needed to know to have many of these pieces I already (roughly) understood click together in a way that makes me understand even the base components better than I did before. I'd love to see something on multi-band equalizer design! a friend recently suggested a room correction circuit when I asked for ideas for something I could practice PCB design with, and I was surprised to not be able to find an IC for it. when I got to trying to make my own from analog components I was very unsure about how to be certain my recombined signal was properly flat. it would be awesome to see such a circuit described with your detail!

[@doggwoggle]

these really are some of the best little circuits AND explanations online. thanks.

[@doggwoggle]

exctept that mine doesn't work 😢

[@clownhands]

Just found your channel, love it!

[@jdarnell123]

Your videos are ALL excellent and VERY well explained! Keep up with the awesome content! You may be The AudioPhool but you're definitely no fool. 🙂

[@VaansWorld]

DUDE you're a legend. Thanks for this!

[@VaansWorld]

Now, how do I adapt it to a single 9V supply with an LFO as a CV? lol THATS the question

[@Sakthi_Boston]

This video was very much informative , Keep the good work going💯👏

[@Warsel]

Excellent explanation, thanks for this video

[@ldezem]

Great content! Thanks!

[@alfredtambunan8961]

Thank you teacher!

[@LousyPainter]

Excellent demonstration! Thanks... Subscribed with bell.

[@9VDC]

Thanks a lot for your videos! Your enthusiasm is absolutely infectious and keeps me watching your videos although i am sometimes a bit behind with my understanding ;-)

[@TheAudioPhool]

You'll get there! I didn't understand a single thing when I started. Just build what you can wrap your head around and the pieces will start to fall into place :)

[@kuro758]

@@TheAudioPhool that's great advice:)

[@MrAlFuture]

Awesome! Thanks!

[@namename8986]

good video man

[@Theineluctable_SOME_CANT]

JUST GREAT!

[@Abihef]

Love The thumb And the idea but I gotta check it tomorrow

[@serjo57]

I had similarly used a transistor driven by an ADSR in my synth in the 70's.

[@NicleT]

Thank you so much!

[@alexloktionoff6833]

Cool idea and awesome video! I've got a question what is the amplitude of the input signal from signal generator sinus? I think it must be totally positive with even positive offset above the ground higher than Base-Emitter pn. Can this schematics be used for AGC? What about distortion ? Thank you for your videos!

[@ahG7na4]

emitter at virtual ground to set the DC operating point (IIUC), this is beautiful

[@AndrewPineiderMusic]

You're my hero

[@TheAudioPhool]

No you're MY hero! Thanks for watching :)

[@mermelada-records]

Great video. Using a 10k trimpot will work as the voltage divider for the audio signal part?

[@majdnbutros4216]

Thank you man you helped alot,, can I design this circuit is multisim?

[@mramonlopez]

Could you replace the physical switch with another transistor acting as switch to use an external CV trigger signal? What changes must be added to support 12V (eurorack)?

[@fer_fdi]

Nice and simple! So, to use external CV instead of the "transient generator", we can connect a buffer to the 1M input resistor (and eliminate capacitor and pot)? And then use a 100k input resistor instead of the 1M?

[@plop-4269]

Thank you !!!!

[@gazzacroy]

very cool :)

[@AhmetKizilay0]

This is very informative. Thank you! Are you using ground as the negative voltage on the op amp?

[@TheAudioPhool]

No the op-amp is powered from +/-9v. drive.google.com/file/d/1y8wOp-qmKGjMiKFT-OwNfj-xdMyugcu3/view?usp=sharing

[@tonigialanze8880]

Hey man thanks a lot for this!! This is really really helpful and informative. I'm fairly new to electronics in general and synth building. I have a question regarding the the voltage divider used for keeping collector around 1V. Are the resistor values right? I tried them on a voltage divider online and I am getting a voltage closer to 0.1V. If I change the 47 ohm to 470 ohms, the voltage is getting closer to 1V which is closer to what we need? I am curious if I am understanding something wrong. Thanks again man.

[@TheAudioPhool]

Hi mate! It should be 220 ohms and 47 ohms. if you check the schematic in the description that has the correct values :) sorry about that! Thanks for watching!! I have a video coming out very soon with a much better VCA so keep your eyes peeled!

[@foobeaglebar]

Another awesome video dude. Gonna try to make something like this on the weekend. Can you do a video about outputting to line level outputs or some other concepts related? I think it'd be attenuation? I built a VCO but output is between -12 and +12 and I'm afraid it's too hot to plug into my stereo.

[@AndrewPineiderMusic]

Video on line level outputs would be helpful... difference between line and mic level or something of that sort

[@foobeaglebar]

@@AndrewPineiderMusic yeah definitely, I haven't been able to find any good videos on building the hardware to handle that stuff

[@TheAudioPhool]

I've put it on my to-do list. :) Thanks for watching!

[@magiclay]

if you put a triangle at input will make triangle shape at output...as any vca?

[@wefyb2]

Yes! It's actually pretty close. There is a small amount of compression due to the signal requiring to broach the limits of the linear region of the transistor, but it does work. You can't expect the output shape to "follow" the input CV extremely closely though, as it doesn't have any way to precisely control the scaling (linear vs exponential, no offset or VCA tuning included, but could be added). Also, worth noting that it works better with small input signals, and using a second opamp to scale the final voltage to your desired levels (or to make maximum at the end equal the input, which is pretty typical for a VCA)

[@blackmandalastudio]

thanksssss for all knolege shared

[@MRsin084]

Hello , can you tell me about That2180LC? What the similar for this ic? Because i can not find this ic in my country. Thank you

[@RickDrift]

So if this were Eurorack, Would the CV input be the 9v at the beginning or the switch right after it?

[@buzzkett]

This effectively how many tremolo circuits work - the cv is an LFO w adjustable gain

[@testbildmuc]

How would you go about adapting this to eurorack voltage levels? (I. E. - +10v signals that are balanced around 0) kinda confused about how the transistor voltages would work out in that context

[@TheAudioPhool]

I nearly added a section to the video about how to shift a signal up that was centered around 0 but it made the video too long! it would still work but you'd find the transistor would conduct for negative voltage swings regardless of the control voltage and essentially just look like a resistor. I actually nearly added a section like this to the video but it made it too long! Basically you wan't to shift the voltage up to be 1/2 of the supply and half the amplitude so it doesn't clip. tinyurl.com/yeld4h78 Here's an example circuit, the first stage cuts the gain by 1/2, the second stage shift the level up to be centered around 1/2 of Vcc. You can do it in one stage but i think it's clearer if i show you like that!

[@TheAudioPhool]

Here's the single stage :) tinyurl.com/yehpwp5u

[@bobdoritique7347]

Merci.

[@skriptico]

Lovely! Would be great to add this to some attiny85 crazy "oscillator" to make some percussions.

[@TheAudioPhool]

Do it and then show me! :)

[@skriptico]

@@TheAudioPhool i will try after the w.e. unemployed bloke here. And complete "eternal" newbie, dont expect anything good ;-)

[@TheAudioPhool]

@@skriptico It's not about good it's about you doing something that you enjoy and creating something new! :)

[@maxbursell3513]

you are so COOL

[@pyrotek45]

can you show a proper schematic of this circuit? i cant seem to recreate it in falstad. it doesn't seem to work? any help here?

[@TheAudioPhool]

Hey mate! My link in the description is broken for some reason so thanks for bringing that to my attention! drive.google.com/file/d/1y8wOp-qmKGjMiKFT-OwNfj-xdMyugcu3/view?usp=sharing Falstad is a bit dodgy sometimes i find it can be a bit weird with some circuits. Try building this one IRL if possible! It's a bit of a quirky circuit but it works really well for percussive sounds :) Let me know how you get on

[@NicleT]

I’m trying this project but the output is noisy af even with the precise components. What am I missing?

[@simonyoungglostog]

I've just stumbled across your channel and subscribed. I'm learning electronics after a 35 year absence and am particularly interested in Audio. Why did you connect to the inverting side of the op-amp?

[@TheAudioPhool]

Glad to have you on board! I have a whole playlist on op-amps which i think you'll find helpful :) Basically when we have negative feedback (feedback to the inverting (-) terminal of the op-amp) the op-amp will try it's best to make both of it's terminals (+ and -) the same. In this context, because we have the + terminal grounded it makes that - terminal look like ground too, which is handy in this circuit because we want the emitter grounded. The op-amp will hold that emitter voltage at 0 volts all the time, without stopping a current from being passed to the output of the circuit. If i put a resistor there instead, the voltage that is developed across that resistor would love the potential difference from the emitter to the base, turning the transistor off! How i like to think of this is that there is a current coming from the emitter towards the op-amp terminal (-), but the op-amp wants to neutralize that current (to make both inputs ground) so it outputs an opposing current to cancel that incoming current, and because we put a resistor in the path, that current becomes a voltage and we take that as the output! That's not exactly what's happening, but it gives you a nice picture of whats going on (i think!). Hope this helps!

[@simonyoungglostog]

@@TheAudioPhool Thank you. So much to remember, relearn and learn anew.

[@richardsstark9825]

I am enjoying your presentations and so far they have worked out. However, i cannot get this one to work. The audio output from the CD4069 is fine, the voltage reading there is 3.8 volts.. The voltage reading from the R4,R5 voltage divider is only 0.08 volts. Based on the resistor values you provide, this is about what it should be. Are these resistor values correct? Also, the output going into the transistor base is only 0.7 volts when I press the button. Is this enough to fully "open" the transistor. Thank you in advance

[@TheAudioPhool]

Hi Richard! That all sounds about right. Those resistor values are correct, the only difference i can see is that the feedback resistor in the current to voltage converter (R6) should be 1k, not 1M. I'll update the schematic.

[@richardsstark9825]

@@TheAudioPhool But, you indicated that you wanted the voltage into the transistor collector to be about 1 volt. Isn't 0.08 volts much too low.

[@TheAudioPhool]

Hi mate! I'm sorry it was like 11pm when we were talking last night:) i read 0.8! The resistor values should be 220 in series with 47 ohms. I think i made the same mistake in the actual video and it still worked, the output from your cd4069 does sound a little bit on the low side too which might be why, is that 3.8v midpoint or 3.8v peak? Are you powering it from 9v or 5v?

[@richardsstark9825]

I just signed up to your patreon account. Good luck! I am using a 9 volt battery which presently is outputting about 8.6 volts. I am measuring the voltage with a multimeter. I do not know if this would be midpoint or peak. I had switched out the 47 ohm resistor with a 1000 ohm resistor. With this the collector voltage is 1.2 volts. The circuit is working to some extent. When I press the button, i get a good audio signal out of the op amp. However, it is working more like a toggle switch. The audio signal comes on when I hold the button down and goes off when I release it. The the capacitor and potentiometer don't seem to be doing anything. I checked the potentiometer and capacitor and both are working OK. I suspect I am doing something wrong but do not know what.

[@aswincvenu3958]

I have been studying cmos ic design for so long I forgot about BJTs

[@re8et355]

Duper!

[@arenotdiy7280]

Expertly explained. Question though: Is it an amplifier? Is there a gain you're getting out, or would voltage controlled attenuator be a more "technically correct" name for the circuit? Really great though, I think this is what I'm going to use in my DIY synth mixer

[@TheAudioPhool]

This is actually a great question! :) Usually in electronics we are concerned about the voltage gain of a circuit, but there is more to it than that! In this circuit we are looking at current, and we have current (and power) gain! I uploaded a video earlier today with a similar circuit that actually does attenuate the voltage a smidge, but amplifies the current significantly, giving it again, current and power gain!

[@arenotdiy7280]

Word! I'm still slogging through the basics of electric math and it's concepts, but sooner or later, current will not be painful to think about. For now, I'll just be thankful it works :)

[@TheAudioPhool]

@@arenotdiy7280 You're doing great! You're obviously getting the hang of it because that was quite an insightful question. Keep going! :)

[@magiclay]

this is a vca or a decay ?

[@thefordfamily2120]

Not sure if this is the place but I had all my knobs replaced gain control lpf bass boost subsonic all them replaced with under valued ones instead of the correct valued ones it doesn't sound right at all but been told wouldn't make a difference sure it does otherwise why have different values

[@coldplay303]

Shouldn't the capacitor and the resistor in series to get a decay output? The voltage will be a constant 9V in your diagram I suppose

[@jonasdaverio9369]

You didn't really explained how the currents Ic or Ie varied. For a fixed Vb, the transistor has to work in saturation region for Ic to depend on what happens on the collector

[@mvuksano]

Maybe a silly question. What speakers do you use? I want to start experimenting with some of the things you're demonstrating here and need to buy some "equipment". All my speakers are Bluetooth. I assume you use some analog speaker to output real audio.

[@TheAudioPhool]

I'm literally just using a pair of crappy USB computer speakers. I have the USB plugged into an old phone charger and crocodile clips attached to the 3.5mm jack to hook into the circuit

[@peschex0d]

This shouldn't be done this way. If your input is square wave, then it is enough to power up a simple common emitter amplifier built with a single transistor from an ADSR source. This way the maximum output vpp is limited by power voltage and therefore will repeat the ADSR envelope. If your input is a complex analog signal like sine wave+pink noise for drums, then you need to decrease it to about 5mv first, then pass through the single transistor common emitter amplifier with the DC collector current controlled by some low 0.1-0.5V bias voltage passed to the transistor base as well. This way the middle point of sine wave is shifted along the transistor base current graph and its curve changes so the output voltage is actually a product of multiplication of bias x signal. The disadvantage is still some clicks at short attacks but you can filter them out, or use balanced/differential two transistor circuit with an opamp output buffer.

[@bonobo9904]

This very simple VCA has only one but sometimes very annoying disadvantage. Control signal mixes with audio signal and will give clicks during every attack. In the early days of electronics such type of modulators could be used for slow attack instruments like strings... but for a synthesizer it is essential to use balanced VCA, like a differential amplifier with current control.

[@andyhowlett2231]

Of course it works, but on analogue signals there will be distortion as the circuit is inherently non-linear.

[@gkdresden]

Well, not really single transistor - single transistor plus op amp (as active devices).

[@misterhat5823]

Using a transistor designed for muting would lead to a much less asymmetric response.

[@kychemclass5850]

I thought MSOFETS were voltage controlled and BJT's were current controlled. :s

[@davidreichert9392]

1001 :)

[@jeffbrown1366]

Hmm, Built it. Didn't work. DC tested without the decay caps, and still doesn't work. 😒

[@OMNI_INFINITY]

Not a single transistor, but thanks though!

[@chrisscott1547]

I think you'll find that a JFET works better ...

[@gkdresden]

The double diode imagination of a transistor is of course wrong. It is also not necessary to keep the collector voltage above the base voltage. It is no problem to go with the collector voltage below the base voltage, like in almost all switching applications and in some amplifier applications as well.

[@selwonk]

Yo man it’s time to start shaving your head and stop trying to pretend those 8 hairs on your head are 15 hairs.

[@densebutton]

Are you always so fckin rude? Seriously, what an unnecessary thing to say, this guy is making great electronics vids, who cares about his hair?

[@Drenov]

It's not a great idea to momentarily short out the battery through the switch