# HFE question

Discussion in 'Homework Help' started by ckaiser813, Jan 21, 2009.

1. ### ckaiser813 Thread Starter Member

Jan 21, 2009
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First time post my lab says this,

assuming hFE is 165 and hfe is 120 for the transistor displayed in the amplifier calculate the voltage gain, AV, of the amplifier.

Anyone have a formula for me, I already calculated VB and IC in the previous questions if that is what I'm looking for.

Thanks

Chris

2. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Can you post a schematic of the amplifier circuit?

hgmjr

3. ### mik3 Senior Member

Feb 4, 2008
4,846
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Then calculate gm=IC/25mV and if the amplifier is a common emitter one the AC voltage gain will be approximately gm*Rc. Rc is the resistor between the collector and positive supply rail. If you want post the circuit to be sure of the result.

4. ### Wendy Moderator

Mar 24, 2008
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Actually Rc also includes the load resistance also. Been bit by that one myself.

5. ### mik3 Senior Member

Feb 4, 2008
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Bill, I assumed that no load exists and the output is just measured from the collector.

6. ### Wendy Moderator

Mar 24, 2008
20,766
2,536
So did I with another thread, and got bit. That's why I put up the schematic (version 2), it helps pin things down.

7. ### mik3 Senior Member

Feb 4, 2008
4,846
63
For the ac voltage gain to equal gm*Rc//RL, you have to bypass Re with a proper valued capacitor otherwise the gain changes. Also, the gain equals gm*Rc//RL for mid band frequencies. For low and very high frequencies the gain reduces due to capacitances in the circuit (coupling and bypass capacitors and the internal capacitances of the transistor).

8. ### Wendy Moderator

Mar 24, 2008
20,766
2,536
Actually, with this circuit gain is the simple Rc/Re. I'm not sure what "gm" is (but am interested). The original drawing had a emitter swamping resistor, but it tends to confuse the issue with the reactance equations, and the gain without the emitter resistor becomes an issue of transistor frequency response and frequency. I assume the capacitor reactance is negligable for the most part on these kinds of designs, after pinning down what frequencies they don't interact much.

I don't mean to hijack this thread however. Want me to draw another one with the capacitor?

If I don't know what gm is, what are the odds the OP does?

9. ### mik3 Senior Member

Feb 4, 2008
4,846
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gm is the transconductance (Ic/Vbe) of the transistor and it has units of Siemens. It is used when you make calculations using the pi model of the transistor.

Last edited: Jan 22, 2009

Mar 24, 2008
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11. ### italo New Member

Nov 20, 2005
205
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Hfe is an inpirical number for an Ic current, is defined as base current to cause Ic to flow by that factor only if the current is within device specification. For hi amps this inpircal number can and will drop substantialy. An Hfe OF 165 at 50 ma can drop to less the 5 at 1-2 amps.

12. ### ckaiser813 Thread Starter Member

Jan 21, 2009
17
0
thanks for the help, i was trying to set up an oscillator with transistors for my course

13. ### Audioguru New Member

Dec 20, 2007
9,411
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If the transistor has enough current gain to be biased correctly then the hFE or hFE has nothing to do with the circuit's voltage gain.
The voltage gain= (look in a tutorial about transistors).