Fuzz Face circuit again V2

[DarkKnight_so]

I'd like to thank everyone for helping me in the last thread that got lost i really understand now alot more
For those who do not know I've been analyzing the fuzz face circuit in detail
Now the next step for me is to do the AC analysis which means drawing a equivalent circuit and calculating the Rin,Rout and Av. Now I know how to do these things with normal one transistor circuits but i don't know how to do it when it comes to 2 transistor amplifiers with the feedback resistor so if anyone would help me get this right or some pointers or maybe a place where i could look it up that would be great

Thank you very much for your help!

[WaveshapeIllusions]

Well... uh... :/ It's complicated. This can be seen as a feedback amplifier, but an imperfect one since open loop gain is low. Also, in the case of AC signals, it is referred to as impedance (Z), rather than R. Impedance is a complex number that includes inductance and capacitance. (There's a lot to it, so I'll just leave it at that. for now.

Z out can be basically seen as the total collector resistance of Q2. There are some other variables but they can mostly be ignored. Feedback doesn't really do much here, as it is from Q2's emitter.

Z in starts to get complicated though. In most cases, it is just the input resistance to ground. But Q1's base resistor isn't connected to ground. It's connected to Q2's emitter. This is the feedback path. Feedback in most cases is used to increase input Z, but it really depends on whether the feedback comes in as series or parallel relative to the input. In this case, it is in parallel, which actually brings down input Z. The change is relative to open loop gain. What's that? Well...

(Side note: Before opamps, a similar circuit to the FF was used. There was the addition of feedback from Q2 vollector to Q1 emitter. In this case, input Z is raised and oupt Z is lowered.)

Open loop gain is how much gain the circuit has without feedback. The feedback loop is open. Closef
d loop gain is probably what you want to know. But first, open loop. Gain is multiplicative, so you can look as the open loop gain as AvQ1xAvQ2. The setting of the gain pot also affects this, as it raises Q2 gain. For this kind of thing though, just an estimate is needed, so just ignore the gain pot for now. Why? Because...

The gain pot also affects feedback ratio. Oh no! This just got a whole lot more complex. See, at min gain pot we can just ignore it since the emitter resistor is in series for AC signals. But at max, that cap effectively shunts all AC signals to ground. Which means... less feedback! So not only does the open loop gain go up, but so does closed loop. Closed loop gain is basically open loop gain multiplied by the feedback ratio, which is how much signal is fed back versus shunted. You probably use it often when setting gain for opamp stages.

But back to the FF. Since there's less feedback, gain goes up. Gain also goes up because of emitter bypassing. So it's a increase in gain multiple ways. However, this is all not really relevant. Why is that? The FF is clipping. When the signal clips, the feedback loop loses control of the signal. The amplifier is effectively open loop when it clips. Of course, the FF doesn't clips all of the signal, it's not full square wave. Part of the signal is amplified normally and then it hits the supply/ground; the signal is open loop. So gain changes based on where the signal is in its cycle as well.

Oh, I forgot something. Remember how input Z includes capacitance and inductance? There's no inductors here, but the FF has an input cap. Caps increase in impedance the lower the frequency is, looking open circuit to DC. So input Z also varies based on frequency.

This is only really the beginning too. There's entire fields of study based on feedback theory. Impedance takes a few classes worth of information.

I hope this got you started though.

[DarkKnight_so]

Well... uh... :/ It's complicated. This can be seen as a feedback amplifier, but an imperfect one since open loop gain is low. Also, in the case of AC signals, it is referred to as impedance (Z), rather than R. Impedance is a complex number that includes inductance and capacitance. (There's a lot to it, so I'll just leave it at that. for now.

Z out can be basically seen as the total collector resistance of Q2. There are some other variables but they can mostly be ignored. Feedback doesn't really do much here, as it is from Q2's emitter.

Z in starts to get complicated though. In most cases, it is just the input resistance to ground. But Q1's base resistor isn't connected to ground. It's connected to Q2's emitter. This is the feedback path. Feedback in most cases is used to increase input Z, but it really depends on whether the feedback comes in as series or parallel relative to the input. In this case, it is in parallel, which actually brings down input Z. The change is relative to open loop gain. What's that? Well...

(Side note: Before opamps, a similar circuit to the FF was used. There was the addition of feedback from Q2 vollector to Q1 emitter. In this case, input Z is raised and oupt Z is lowered.)

Open loop gain is how much gain the circuit has without feedback. The feedback loop is open. Closef
d loop gain is probably what you want to know. But first, open loop. Gain is multiplicative, so you can look as the open loop gain as AvQ1xAvQ2. The setting of the gain pot also affects this, as it raises Q2 gain. For this kind of thing though, just an estimate is needed, so just ignore the gain pot for now. Why? Because...

The gain pot also affects feedback ratio. Oh no! This just got a whole lot more complex. See, at min gain pot we can just ignore it since the emitter resistor is in series for AC signals. But at max, that cap effectively shunts all AC signals to ground. Which means... less feedback! So not only does the open loop gain go up, but so does closed loop. Closed loop gain is basically open loop gain multiplied by the feedback ratio, which is how much signal is fed back versus shunted. You probably use it often when setting gain for opamp stages.

But back to the FF. Since there's less feedback, gain goes up. Gain also goes up because of emitter bypassing. So it's a increase in gain multiple ways. However, this is all not really relevant. Why is that? The FF is clipping. When the signal clips, the feedback loop loses control of the signal. The amplifier is effectively open loop when it clips. Of course, the FF doesn't clips all of the signal, it's not full square wave. Part of the signal is amplified normally and then it hits the supply/ground; the signal is open loop. So gain changes based on where the signal is in its cycle as well.

Oh, I forgot something. Remember how input Z includes capacitance and inductance? There's no inductors here, but the FF has an input cap. Caps increase in impedance the lower the frequency is, looking open circuit to DC. So input Z also varies based on frequency.

This is only really the beginning too. There's entire fields of study based on feedback theory. Impedance takes a few classes worth of information.

I hope this got you started though.

Thank you very much
Well I was thinking in setting the gain pot to a middle position lets say 500R since Im trying to get some correct equations and later itl be easy to calculate the Av when the gain pot changes.
Now if I understand right to calculate the voltage gain Av I first need to calculate the input impedance and i was doing it like this. first calculate the hie which is 0.025/Ib1 which gives me around 7k ohm. Now I messured the input and i got around 1.5k ohm as input resistance. I read somewhere that since its a feedback amplifier i need to divide that hie with (1+B*hie) where B is the feedback factor. Now I know i need to calculate the B somehow and thats where my problem is.

About Av i know to get the right Av with feedback i need to calculate the Av open loop(which means without the R3 feedback resistor, IF IM CORRECT) and then use the same equation Av/(1+B*Av) to get the feedback voltage gain. Now my problem again is the feedback factor. I have to find a way to calculate it i know just that i can get it from some resistors relations but i dont know how to determine that.
Please correct me if Im wrong in any way

Thank you for your help!

[Kesh]

Z out can be basically seen as the total collector resistance of Q2. There are some other variables but they can mostly be ignored.

Are you sure? I thought Z out was lower due to tapping the output close to V++ instead of at the collector, and with a related loss of gain.

[WaveshapeIllusions]

You know, I'm not entirely sure. I would assume so, since the lower resistor is still part of the collector load. Which means it still sets the effective gain that Q2 is operating at and that it will still limit the current that the collector can supply.

[Kesh]

You know, I'm not entirely sure. I would assume so, since the lower resistor is still part of the collector load. Which means it still sets the effective gain that Q2 is operating at and that it will still limit the current that the collector can supply.

But current is supplied by the battery/power source, not the collector. Collector current just determines voltage over Rc.

Think about how an output loads the collector. It will be 8K2 + (470R||Output_Load). So any output load will barely load the collector, it will always be at least 8k2. However it will, in parallel with the 470R, act as a voltage divider with 8k2.

We really need someone who knows what they're talking about in here.