37. Class C Amplifiers

This is an introduction to the Class C amplifier. The classes theoretical efficiency, linearity, biasing, and applications are briefly covered. I play a little fast and lose with some of the math for simplification. As usual, a sample circuit is built and it's characteristics are measured to compare with the presented theory.

[@NajCharfeddine]

what is the point of this amplifier? why not just make an oscillator?

[@zhangjin7179]

246 nano watt or 246 micro watt?

[@MyProjectsTV]

I have no idea how you got the values for the clamper circuit. A bit more calculation with the 5Mhz and the values of capacitor and resistor would've been nice. I can't recreate that. For me it's not 10 times the input. Please help.

[@sidahmedbensaad1838]

Thank you for a great explanation from algeria north africa.

[@mn.naseri]

I have a video transmitter. I want to increase the range of the transmitter. Which amplifier should I use for the UHF band?

[@shaggygoooxide]

Nice explanation! One quick question: how did you choose the value 1k for your efficiency calculation?

[@fantozzi30]

At 26:33, shouldn't it be microwatts instead of nanowatts?

[@milodelpozo7727]

its 240 mW not nW, dont forget to plug in the correct units

[@zhangjin7179]

there is an error in your calculation!!!

[@electronicsexplained287]

@ 5:10 , how do we go from negative to positive while the cap is discharging? Isn't Vb or the upper resistor pin connected the base o transistor supposed to be at higher potential than the GND so + to - meaning a positive 0.3 Volts bias? What am I not getting in here?

[@researchandbuild1751]

I have trouble when trying to simulate this circuit in LTSpice it just doesn't seem to give me a nice sine wave output. Hmm I changed my timestep in LTSpice to 1nanosecond and now I get a sine wave, but it looks like every so often the sine wave ends up "building up" to some larger value before settling down again. But it looks a lot better. Gotta learn to use the tools better.

[@researchandbuild1751]

Thanks for the extra math, i was having trouble figuring out the collector current

[@jmaguilarr]

Thx teacher

[@habtamusium8646]

thanks !

[@he1py]

Amazing explanation! From 7X3TL

[@user-su5sq5ib3i]

Again, thank you!!!

[@TheOffsetVolt]

My pleasure!

[@arthursgarage6550]

So would this amplifier work for an fm circuit? I'm confused how this would double the frequency of an fm wave since there would also be frequency deviation.

[@TheOffsetVolt]

Yes it will. You use the class c to double the carrier frequency and then modulate it - no worries about doubling the modulation this way! 73's

[@arthursgarage6550]

@@TheOffsetVolt could it also double the frequency deviation as well?

[@_Junkers]

Keeping the pulse width of the input signal seems to benefit efficiency, but what about the signal strength from the inductor?

[@TheOffsetVolt]

Hello again, The inductors Q is a factor. Any internal resistance needlessly wastes energy and we will always be stuck with some R. So, if you are looking for top notch efficiency, use a high Q inductor, you might see and extra percent or so.

[@sayantanmaiti2513]

I have a transmitter that has a input audio source of peak frequency 20Khz( as usual) and I fed that into a class C which is at 97MHz resonant frequency...and still that works fine...

[@TheOffsetVolt]

Hello, You probably have parasitics causing the oscillating in the transistor. A small ferrite bead on the base might eliminate them.

[@researchandbuild1751]

Class C amplifiers can resonate at multiples of the input frequency

[@Starbuckin]

Great video thanks for sharing!

[@khimroy3958]

Thanks 👽👍

[@TheOffsetVolt]

Spasibo, glad you liked it.

[@oneandrew7767]

Thanks for clearing this up in a simple easy to understand way.

[@TheOffsetVolt]

Glad it helped.

[@mdmasumbillah1796]

Thanks a lot.

[@dikshitgehlot4120]

It helped alot

[@electromatic2014]

Could not be more clear, amazing, thanks a lot

[@PaulTeogalbo]

What transistor are you using?

[@TheOffsetVolt]

2N3904

[@PaulTeogalbo]

NPN? or PNP?

[@TheOffsetVolt]

The 2N3904 is a NPN. Just about any small signal NPN would work.

[@PaulTeogalbo]

I'm working on a class c amplifier project for school. My professor explained that the input frequency (high pass filter) at the base must equal the resonant frequency (tank circuit) at the collector of the transistor in order to get the theoretical efficiency, I would need to match the frequencies. Is that correct? What would be a good frequency to use?

[@sarvarsabiroff4722]

@@PaulTeogalbo can you share me your term paper? please . i caould not understand how to calculate this cascade if input frequency is Low ((