Class A voltage amplifier circuit w/o caps or inductors - diyAudio
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Old 2nd April 2011, 09:01 PM   #1
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Default Class A voltage amplifier circuit w/o caps or inductors

Hi, I'm designing a "minimalist" amp and I need some sort of circuit that will amplify voltage for the input stage without nasty coupling capacitors and operates in class A mode. No current will be flowing out of the circuit.

Tried a simple common emitter circuit but seems like you need the input capacitor for it to work properly.

I know I could use a class AB push-pull configuration but darn it I want it pure class A.
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Old 2nd April 2011, 09:54 PM   #2
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You can do it (or get close) if you use a split level power supply.
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Old 2nd April 2011, 10:17 PM   #3
DF96 is online now DF96  England
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You could try using nice coupling capacitors. Or, as it will be running with no output load (so no output current), just don't bother to attach the input signal.

You could use an op-amp with a split supply, and accept any DC offset it produces. That way, at least all those nasty Cs and Ls will be hidden away in the PSU.
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Old 3rd April 2011, 02:34 AM   #4
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Don't attach the input signal?
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Old 3rd April 2011, 04:33 AM   #5
djk is offline djk
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You're not going to draw any output current, why would you need an input signal?
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Old 3rd April 2011, 06:26 AM   #6
MiiB is offline MiiB  Denmark
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The funny this is that there's is no design that excludes capacitors...If you don't use them as coupling in the input and out put (input cap can sometimes be omitted), then the PSU caps are right in the signal path...as they support the GND end of the signal...
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Old 3rd April 2011, 07:11 AM   #7
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by cuallito View Post
Hi, I'm designing a "minimalist" amp and I need some sort of circuit that will amplify voltage for the input stage without nasty coupling capacitors and operates in class A mode. No current will be flowing out of the circuit.

Tried a simple common emitter circuit but seems like you need the input capacitor for it to work properly.

I know I could use a class AB push-pull configuration but darn it I want it pure class A.
I don't understand "no current will be flowing out of the circuit" !

So what do you want this amp to drive ?
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Old 3rd April 2011, 08:35 AM   #8
Elvee is offline Elvee  Belgium
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Here is a 20dB class A amplifier, without inductors or capacitors:
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File Type: gif 20dBamp.gif (7.7 KB, 582 views)
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Old 3rd April 2011, 08:58 AM   #9
djk is offline djk
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The CD4069UB looks like it is class B above 0.25A at 25C.

http://focus.ti.com/lit/ds/symlink/cd4069ub.pdf
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Old 3rd April 2011, 10:44 AM   #10
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by djk View Post
The CD4069UB looks like it is class B above 0.25A at 25C.

http://focus.ti.com/lit/ds/symlink/cd4069ub.pdf
Where did you see that?

From the characteristics, I see that output stage remains class A from 2.5 to 7.5V for 10V supply.
And the output current can also reach 1.5mA without too much difficulty.
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