jason9

modified 7 years ago

Small Signal Amplifier

326

06:52:56

This amplifier has a gain of 42.3dB (130V/V) for any signal no matter how small because the transistor is already biased and the signal doesn’t have to be large enough to bias the transistor on it’s own like in a class-C amplifier. The capacitor at the top is to limit the high-frequency response. This amplifier has some harmonic distortion, but that can be reduced by giving it a smaller signal so that the output doesn’t distort as much or by using a different design.

Edit: @hurz had brought up the point that because the upper frequency limiting capacitor fed directly to the base of the transistor, which directly connects to the input through a capacitor, it not only loaded down on the input but it was very sensitive to input resistance. To fix that, I repositioned the capacitor so that it did not connect to the input and was not affected by input resistance, but input resistance still reduces the gain.

Edit2: Since @hurz didn’t like how the 4.7nF capacitor connected to the emitter of the transistor so that the 4.7mF capacitor acts a a direct route to ground, I moved it so that it was between the collector of the transistor and actual ground.

published 7 years ago

hurz

7 years ago

I wont like to have an impact from the audio source to my amplifier. This one has a dramatic sensebility of output impedance of audio source, so even with 600 Ohm the amplifier bandwidth breaks down. http://everycircuit.com/circuit/6740907826348032 this is of massiv negative feedback to the base which is directly coupled to the audio source.

zorgrian

7 years ago

Yeah, you need another stage that can actually drive something, without affecting your little amplifier. Also, this is not the only topology that you might use to obtain high gain from low signal input. Also, Jason, in your post on 741 ,op amps, you say something like "if i remember correctly" ... I thought you claimed to be a juvenile???

jason9

7 years ago

Ever heard of the internet? Wikipedia is pretty much my favorite website.

hurz

7 years ago

???

jason9

7 years ago

@hurz, that problem can be fixed by adding a resistor (like 1kOhm) to the input and readjusting the bandwidth limiting capacitors (10uF, 1mF, and 100pF).

hurz

7 years ago

For any problem, there is a solution. But first, you have to see the problem!

jason9

7 years ago

What’s your point? That I don’t see the problem? What is it then?

hurz

7 years ago

I just told you there is a problem, I dont asked you how to solve it. Its your problem not my one. Fix it and showup with a new revision.

hurz

7 years ago

As response to you description update: 0.0047 cap against 2200 cap is like a sandcorn against the Washington Monument, nothing!

jason9

7 years ago

The 2.2mF capacitor is effectively a direct route to ground causing the 4.7nF capacitor to load down the amplifier at higher frequencies.

hurz

7 years ago

And as capacitive divider it gives you a ratio of 1:468000 so almost nothing

jason9

7 years ago

That ultra high ratio is why the 2.2mF capacitor is practically a direct route to ground. It would make no difference if you connected the 4.7nF capacitor straight to ground. I simply connected it to the 2.2mF capacitor because it was more compact, looked better, and made no difference.

hurz

7 years ago

But the little 4.7nF does not cause any noticeable feedback against 2200mF!

jason9

7 years ago

It’s not supposed to. The 2.2mF is supposed to act as a route to ground. Would it help if I just connected it straight to ground instead?

hurz

7 years ago

That is what you have changed after you added the @hurz comment in your description. You see what chaos you made. Why not come up with a new circuit with a solution of what i have said initial? So how to overcome the bandwidth break down with some resistance of the audio source?

jason9

7 years ago

There. I moved the 4.7nF capacitor. Now the bandwidth should remain unchanged or possibly increased on the low end by input resistance. Not that it was any different before.

hurz

7 years ago

Thats no surprise. I know it does not make any difference to put it directly to ground. But is this related to my first comment to this circuit about bandwidth breakdown?

hurz

7 years ago

@jason9, your documentation is missing the protocoll what have YOU done to solve the issue. We started here: http://everycircuit.com/circuit/5090450041208832 and I told you the bandwidth does depend on the AC audio source impedance. Check yourself and tune up the 1 Ohm resistor and see the frequency response.

jason9

7 years ago

The only frequency response change exhibited by this circuits when audio source resistance is added is a bit of increase in the lower end causing it to load let in lower frequencies more, but it’s not significant much of an effect.

hurz

7 years ago

Jason, have you checked my link and tested the 1 Ohm resistor? See whats happens to the higher corner frequency!

jason9

7 years ago

That’s not the circuit I have here. It used to be like that, but I changed it and now the problem is fixed.

hurz

7 years ago

Yes, but grounding your feedback cap, what it was original, wont help you to use it as a feedback to lower the harmonic distortion.

hurz

7 years ago

So what was your intention to have it before at the base?

jason9

7 years ago

My intention to have it before at the base was so that it could provide the same function it does now, I simply hadn’t realized it that it was putting a load on the input and that because of that the input resistance was a huge factor.

hurz

7 years ago

Ahh, ok.

hurz

7 years ago

BTW, i dont like the way you comment out of context in your description.

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