HFE question

ckaiser813

Joined Jan 21, 2009
17
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

hgmjr

Joined Jan 28, 2005
9,029
Can you post a schematic of the amplifier circuit?

hgmjr

mik3

Joined Feb 4, 2008
4,846
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.

Wendy

Joined Mar 24, 2008
21,840
Actually Rc also includes the load resistance also. Been bit by that one myself. mik3

Joined Feb 4, 2008
4,846
Bill, I assumed that no load exists and the output is just measured from the collector.

Wendy

Joined Mar 24, 2008
21,840
So did I with another thread, and got bit. That's why I put up the schematic (version 2), it helps pin things down.

mik3

Joined Feb 4, 2008
4,846
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).

Wendy

Joined Mar 24, 2008
21,840
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?

mik3

Joined Feb 4, 2008
4,846
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.

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Wendy

Joined Mar 24, 2008
21,840 italo

Joined Nov 20, 2005
205
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.

ckaiser813

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

Audioguru

Joined Dec 20, 2007
11,251
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).