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#1
11-15-2011, 12:33 AM
 jegues Senior Member Join Date: Sep 2010 Posts: 728
Series-Shunt Feedback Amplifier

I think I managed to get part a), but I'm stuck on part b).

I wrote all the equations I could think of in the circuit but I'm still a little stuck as to actually solving the DC emitter currents. Is there an assumption I can make about any of the other currents?

For example,

Can I assume,

$I_{c1} \approx I_{1}$

Is this acceptable? Or should I be able to solve for everything?

I feel like I need to solve 1 value or make an assumption in order to get things rolling.

Thanks again!
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#2
11-15-2011, 02:09 AM
 t_n_k Senior Member Join Date: Mar 2009 Posts: 4,807

The closed loop gain should be ~100.
#3
11-15-2011, 02:33 AM
 t_n_k Senior Member Join Date: Mar 2009 Posts: 4,807

For the currents you can do an incremental approach.

First assume IB1≈0.

Then calculate the current in R1=0.7/1k=0.7mA. Hence current in R2=0.7mA and hence it follows that IE2=1.7mA.

Thus IB2=1.7mA/101=16.8uA.

This makes IC1=100uA-16.8uA=83.2uA. Thus IB1 would be better approximated as 832nA.

You could redo the process now using IB1=832nA rather than IB1=0A. But I doubt things would change that much.
#4
11-15-2011, 02:58 AM
 jegues Senior Member Join Date: Sep 2010 Posts: 728

Quote:
 Originally Posted by t_n_k The closed loop gain should be ~100.
How did you draw this conclusion?

I've been reading in the book how to estimate the closed loop gain when we have a large loop gain but obviously I found the wrong relationship.

My reasoning was as follows,

$A\beta \rightarrow \infty$

$A_{f} = \frac{V_{o}}{V_{s}}= \frac{A}{1+A\beta} \Rightarrow \frac{A}{A\beta} = \frac{1}{\beta}$
#5
11-15-2011, 04:24 AM
 t_n_k Senior Member Join Date: Mar 2009 Posts: 4,807

Begging your pardon - it should be ~10 rather than ~100. I slipped an order of magnitude in my original "back of the envelope" estimate.

Basically, I'm looking at what happens at the output emitter (E2) as the input varies.

With the input 0V then VB1≈0.7V and hence VE2≈7.7V.

If the input increases from 0V to +10mV then VB1=0.71 V and hence

VE2=7.1+0.71=7.81V.

So the VE2 has changed by (7.81-7.7) 0.11V for a 10mV input change, which equates to a gain of Av=0.11/0.01=11.

If the input is -10mV then VB1=0.69V and VE2=6.9+0.69=7.59V

So for a change from 0V to -10mV the E2 voltage changes by 7.7-7.59=0.11V - again indicating a small signal gain of ~11.
#6
11-15-2011, 04:29 AM
 t_n_k Senior Member Join Date: Mar 2009 Posts: 4,807

I can run a simulation to check the result if you don't have access to simulation software.
#7
11-15-2011, 04:45 AM
 t_n_k Senior Member Join Date: Mar 2009 Posts: 4,807

Using the idea of feedback you can come up with the gain as follows.

If the open loop gain is Avo then

$V_o=A_{vo}$V_s-\frac{R_1}{(R_1+R_2)}V_o$$

Re-arranging this gives

$V_o$1+A_{vo}\frac{R_1}{(R_1+R_2)}$=A_{vo}V_s$

or

$V_o$\frac{1}{A_{vo}}+\frac{R_1}{(R_1+R_2)}$=V_s$

And if Avo is very large then

$V_o$\frac{R_1}{(R_1+R_2)}$=V_s$

and finally

$A_f=\frac{V_o}{V_s}=\frac{(R_1+R_2)}{R_1}$

So Af=11 with the given values looks consistent.

Perhaps you have confused the transistor current gain beta values with the feedback beta factor.

i.e.

$\beta=\frac{R_1}{(R_1+R_2)}$

Last edited by t_n_k; 11-15-2011 at 04:52 AM.
#8
11-15-2011, 01:27 PM
 Jony130 Senior Member Join Date: Feb 2009 Location: Poland/Wroclaw Posts: 2,287

I was bored today so I decided to find open loop voltage gain.

I use this diagram

And I assume Ic1 = 100uA; Ic2 = 1.6mA

re1 = 26mV/100uA = 260Ω

re2 = 26mV/1.6mA = 16.25Ω

So the voltage gain of the first BJT is equal

$A1 = \frac{Rc}{re} = \frac{( hfe+1)*(re2 + R1+R2)}{re1 + \frac{R1||R2}{Hfe+1} } = 4.13K$

And I'm not sure whether this (R1||R2)/(Hfe +1) should be take into account.

Voltage gain for Q2 is equal

$A2 =\frac{Re}{re+Re} = \frac{R1+R2}{re + R1+R2} = 0.998$

So open loop gain

Ao = A1 * A2 = 4.12KV/V

The input resistance

$Rin = {re1 + \frac{R1||R2}{Hfe+1} } = 269\Omega$

So after we close the feedback loop the voltage gain will drop to:

Af = Ao/ ( 1 + Ao*β ) = 10.97 V/V

and

Rin_f = Rin* = 101KΩ

Rout_f = (R1+R2) /
(1+ Ao*β) = 29.3Ω But I also not sure about that.

So what do you think about my simplified analysis?
 The Following User Says Thank You to Jony130 For This Useful Post: t_n_k (11-15-2011)
#9
11-15-2011, 08:07 PM
 t_n_k Senior Member Join Date: Mar 2009 Posts: 4,807

Thanks for your useful analysis Jony130.

On the matter of which components to include in the open loop AC gain analysis one could perhaps assume that the feedback is 'killed' by shunting Q1 base to ground with a large capacitance. So the DC conditions would be 'preserved' but the AC gain would presumably more closely approach the open loop case.

Would that mean you then dispense with the (R1||R2)/(Hfe +1) term and replace the R1+R2 combination at Q2 emitter with just R2? Presumably yes.

In any event the open loop gain is highly dependent on the Q2 hfe value.

The addition of the load resistance RL to the circuit will also have an effect.
#10
11-15-2011, 11:41 PM
 jegues Senior Member Join Date: Sep 2010 Posts: 728

Quote:
 Originally Posted by t_n_k Perhaps you have confused the transistor current gain beta values with the feedback beta factor. $\beta=\frac{R_1}{(R_1+R_2)}$
I think this the source of my confusion.

So the $\beta_{1}, \beta_{2}$ are the transistor current gain beta values NOT the feedback beta factor.

Thanks again!

Last edited by jegues; 11-16-2011 at 12:07 AM.

 Tags amplifier, feedback, seriesshunt

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