# Transistor o/p signal voltage swing

Discussion in 'General Electronics Discussion' started by samy555, May 13, 2010.

1. ### samy555

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May 11, 2010
Hi aLL
For the following Fixed bias emitter stablized circuit

the voltage swing from Vcc to Isat RE; that is the ac signal is carried on VC dc voltage and swing up from Vc to Vcc and down from Vc to Isat RE.
My question about the second fixed bias circuit that follows

is the voltage swing from VCC up to zero down?
Is it from VCC up to Isat re down, where re is 0.026/IE?
Another question: For the second circuit If Rc= 1.5K and RB=540k
what the maximum peak-to-peak i/p signal voltage that can be amplified without distortion.
thank you alot.

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Homework?

Since the bias is not done with a voltage divider, in the second case (and in a slightly lesser extent the first) the gain of the transistor determines the resulting Vc.

Since we don't know the gain of the transistor, we can't say much.

3. ### samy555

63
0
May 11, 2010
I'm not understand you!
for the second circuit Vcc= 3 volts, beta = 230, 2N3904 transistor, VBE=0.67V and IC =1mA.
its not h/w i study it alone
thanx

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Work through this, it should help.

If you're really keen, get a copy of The Art of Electronics, which has typically excellent reviews. I think it's the sort of book you can read and understand at home.

They take a very practical (rather than overly theoretical) approach.

5. ### samy555

63
0
May 11, 2010
Thank you Steve

6. ### davennModerator

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Sep 5, 2009
Thanks Steve, even for me ... as some one who has worked in electronics for over 40 yrs
I still learn new things. It had me bouncing around various www pages via google but I couldnt find an answer to something ....

that page you linked to and others stated...
Class AB Operation
The Class AB Amplifier is a compromise between the Class A and the Class B configurations above. While Class AB operation still uses two complementary transistors in its output stage a very small biasing voltage is applied to the Base of the transistor to bias it close to the Cut-off region when no input signal is present.

Now I mainly work with RF amplifiers and the statement above doesnt gel.
As for most linear RF amps are running in class AB but there is only one device
now of course we also use class C for FM transmitters
but my Q is really the previous line and how that works with only one device ... Yes it is being biased
but the transistor is conducting both halves of the cycle. So I would think its really class A, yet they call it AB

Dave
VK2TDN

Last edited: May 16, 2010
7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
I'm pretty sure that in an RF amplifier where you are driving a tuned circuit you can get away with pumping energy into it for less than the full cycle and relying on the tuned circuit to "fill in the blanks" as it were.

It's pretty much the explanation for class C where (as I recall) the transistor is biased well into cut-off.

RF is not something I play with though