Electronics > Beginners
Question regarding a simple amplifier circuit
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sentry:
Okay, so I understand that RE is used to stabilize the transresistance instability in the transistor to combat distortion by lowering the amplification gain for the DC signal. CE is used to bypass RE for the AC signal and thereby obtaining the full gain of the amplification, but only for the AC signal. What I don't understand is why this construction doesn't distort the AC signal because its unaffected by the negative feedback caused by RE? I would expect the AC signal to also be sensitive to the small hFE fluctuation caused by temperatur and current changes?

David Hess:
It does distort the AC signal to the extent that the transistor's own transconductance changes with current which changes with output voltage level.

This configuration is used to provide maximum AC gain while maintaining DC stability which would otherwise allow the transistor to suffer thermal runaway.
Zero999:
The hFE makes no difference to the transistor circuit's voltage gain, only its current gain. It's easy to build a transistor circuit to have a far higher voltage gain, than current gain.

RE is required because it would be virtually impossible to bias the transistor so roughly half the supply voltage appears across RC in its quiescent state, without it. VBE is too variable and the transistor has too higher transconductance to allow that to happen.

See link below for more information:
http://www.ee.ic.ac.uk/pcheung/teaching/aero2_signals&systems/transistor%20circuit%20notes.pdf

The distortion can be avoided, at the cost of reduced gain by removing CE or adding a resistor in series with it.
sentry:
Okay, so I constructed this simple amplifier and I was expecting to see the output signal phase shifted 180o from the input signal, but I only see 80o. Is that because of the output capacitor? I'm using a BC548 NPN transistor in my circuit.
Zero999:

--- Quote from: sentry on August 26, 2018, 04:36:53 pm ---Okay, so I constructed this simple amplifier and I was expecting to see the output signal phase shifted 180o from the input signal, but I only see 80o. Is that because of the output capacitor? I'm using a BC548 NPN transistor in my circuit.

--- End quote ---
It might be.

What frequency did you test it with and what's the load impedance?
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