NPN transistor as an amplifier

[Lalo79]

alec_t,
How are you calculating the gain? I may have a different interpretation of how to calculate the gain since the schamatic giving in post #16 is different from what I have seen before. This is new stuff to me and do not know how to interpret it.
Thanks

 

[alec_t]

Gain = voltage out (pk to pk) / voltage in (pk to pk)

 

[Lalo79]

On the circuits that I am creating, the second stage, which is a Common Collector, I a gain of about 6. How or what resistors determine the gain. I am also face with the problem of the bottom swing having more gain the the top swing and in some cases saturating. What do I do? I was thinking on adding a third stage but with a gain of 5 for a total gain of 30 (5x6). I already look at some formulas relating to gain but for some reason I am not getting any good results when i aply them.

 

[alec_t]

Can you post your circuit?
In theory, gain for an amplifier stage using a single transistor of infinite beta can generally be calculated from a formula as a function of two resistors. However, beta is never infinite and the resistors may be loaded by impedances of neighbouring stages, so reality rarely agrees with theory.

 

[Lalo79]

Amplifier circuit

Here is the circuit that I am simulating. When I simulate the last two stages using an input of 1 mVpk I get a gain of about 6 but if I add another stage (as in this circuit) then the circuit saturates at both ends and there is a gain of about 45 on the top swing and a gain of about 80 on the bottom swing. That is really weird.

 

[audioguru]

I wish you would turn off the chicken-pox dots on your schematic.
You should look at DC voltages in your SIM:
1) The first two transistors are saturated so cannot amplify anything.
2) The output transistor does not have enough base current.
3) Why use a class-A circuit instead of a class-AB circuit??

 

[alec_t]

Here's a circuit which (at least when simulated) seems to meet the specs:-
Vsupply = 3V
V Gain = 30 (Vin = 10mV pk-to-pk 1kHz, Vout = 300 mV pk-to-pk))
Zin ~ 140k
Low distortion (2nd harmonic ~ 59 dB down, 3rd harmonic ~ 35dB down)