Tuesday, 26 June 2018

circuit analysis - Calculating the transresistance in a multistage voltage-shunt(shunt-shunt) feedback amplifier


I've been trying to analyze the following circuit using whatever books and resources I have to hand:


schematic



simulate this circuit – Schematic created using CircuitLab


I've identified the topology as a voltage-shunt feedback amplifier (is this correct?) It looks as if the voltage is sampled at the output and the feedback current is subtracted (anti-phase) from the source, therefore voltage-shunt.


Any examples I've seen for this type of topology are either the collector feedback single stage amp or and classic inverting op-amp circuit. But, I haven't come across any multi-stage examples on which to base my analysis.


Applying the usual rules (short Vo to find input, short Vi to find output) to find the circuit to calculate the open loop gain for this topology, I get the following:


schematic


simulate this circuit


Using AC analysis for each stage, I calculate the trans-resistance for stage 2 (Rm2) as 27k, given by:


Note: I've used certain sensible (I think) simplifications here.


Rm2=VoIb2=hFeR5=(100)(270)=27k


and the trans-resistance of stage 1 as:



Rm1=Vo1Is=Ic1RLIs=Ic1Ib1Ib1IsRL=hFeIb1IsRL=73kwhereRL=hFeR5R1=27k820=795andIb1=IsRiRi+hFerewhereRi=R2R3R5


I'm not sure I've gone about this the right way, but it seems that multiplying the two trans-resistance values is not the way to go (like you'd do for multi-stage voltage-series amps).


So, my question is: how do I proceed to get the open-loop trans-resistance in this (or any) multi-stage voltage-shunt configuration?


UPDATE: Thinking about it, I could use the same method as with a current-shunt feedback amplifier and express:


VoIs=Ib1IsIc1Ib1Ib2Ic1VoIb2=Ib1IshFeIb2Ic1VoIb2


but I'm just not sure.



Answer



I think that the voltage gain cannot be larger than R2/R5 in this case.


If I do more "traditional" analysis (I'm not familiar with the TS method)


schematic



simulate this circuit – Schematic created using CircuitLab


I get the voltage gain equal to :


AV=R1re1R5||R6R5||R6+re2R1re138ICR1386.2mA820Ω193V/V


And now using the Miller effect I can find the Voltage gain of the whole amplifier


schematic


simulate this circuit


AV2=R3||R21+AV||rπR5+R3||R21+AV||rπAV5V/V


Additional I assumed $ r_\pi =(\beta +1) r_e = (100 +1) \cdot \frac{26mV}{6.2mA} = 423\Omega$


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