
Homework Help Stuck on a textbook question or coursework? Cramming for a test and need help understanding something? Post your questions and attempts here and let others help. 

Thread Tools  Display Modes 
#1




SeriesShunt 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, 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! 
#2




The closed loop gain should be ~100.

#3




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=100uA16.8uA=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




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, 
#5




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 (7.817.7) 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 7.77.59=0.11V  again indicating a small signal gain of ~11. 
#6




I can run a simulation to check the result if you don't have access to simulation software.

#7




Using the idea of feedback you can come up with the gain as follows.
If the open loop gain is Avo then Rearranging this gives or And if Avo is very large then and finally 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. Last edited by t_n_k; 11152011 at 04:52 AM. 
#8




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 And I'm not sure whether this (R1R2)/(Hfe +1) should be take into account. Voltage gain for Q2 is equal So open loop gain Ao = A1 * A2 = 4.12KV/V The input resistance 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 (11152011) 
#9




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 (R1R2)/(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




Quote:
So the are the transistor current gain beta values NOT the feedback beta factor. Thanks again! Last edited by jegues; 11162011 at 12:07 AM. 
Tags 
amplifier, feedback, seriesshunt 
Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Amplifier Feedback  Shagon  General Electronics Chat  4  06152011 05:46 PM 
(PNP) Collector Feedback Bias Amplifier Circuit  huntlk  Homework Help  1  02222010 08:29 PM 
Two stage BJT amplifier with feedback  The Electrician  General Electronics Chat  57  08272009 04:01 AM 
common emitter amplifier circuit using feedback bi  den  Homework Help  14  03042006 04:58 PM 
Thread Tools  
Display Modes  

