# BJT h-parameter gain problem

Discussion in 'Homework Help' started by Stereoblind, May 6, 2015.

1. ### Stereoblind Thread Starter New Member

Mar 10, 2015
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Hey guys,

I've currently been trying to work on this question and establish the voltage/current gain of my h-parameter model. I get the correct answer for the current gain but can't get the proof right for my voltage gain. I've tried it a couple of ways and the way I've attached here seems to be the closest I can get. Can anyone see where I've gone wrong? Thanks.

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2. ### Stereoblind Thread Starter New Member

Mar 10, 2015
14
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Ahhhh I see what I have done I used the wrong output voltage value in the first stage. Stupid mistake.

3. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,300
335
Do you get the result that Gv is gm*RL/2? That seems a little strange. Is gm the transconductance of a single transistor?

4. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Why? As I remember correctly the voltage gain for long tail pair is two times smaller than the CE amp voltage gain.
CE gain is gm*Rc so for LTP the gain must be equal to (gm*Rc)/2. Where gm is transconductance of a single transistor.

5. ### Stereoblind Thread Starter New Member

Mar 10, 2015
14
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Sorry I realised I didn't upload the circuit diagram. Yeah I managed to get it equal to gm*RL/2. I've attached a picture of how I achieved that . Yeah that's the transconductance of a single transistor.

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6. ### Stereoblind Thread Starter New Member

Mar 10, 2015
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Also I apologise because I think my initial diagram is a bit poorly labelled! Also RE >> hie/hfe so we assumed all the current has gone into hie/hfe

7. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,300
335
This problem has an unusual mix of H parameters and Y parameters. The student is asked to consider characteristics of the individual transistors, and also the overall amplifier. I thought that the parameter gm was referring to the overall amplifier treated as a two-port, but I wasn't sure.