Bjt transistor amplifier

Thread Starter

Zeeus

Joined Apr 17, 2019
540
trying to understand BJT... Someone please assist

Firstly, please can someone orient the transistors in spice so it's easier to understand..Thanks

Did not try to make the push pull better as was only interested in seeing the amplifier part
:

Tried to make the circuit...Result is different from spice and not sure I made a mistake...
The voltage at base of Q1 was same as at base of Q2 (feedback yh? Always good to see it checks out :) ) but now the voltage at the output of the push pull is about -10v which is no good...Increasing R3 reduces the voltage...What am I missing??

Removed Q4 and Q5 and used 1mA on Q1 and connected collector of Q2 to -12V : here everything checks out..voltage at ..Gain was about 100....Just trying to use Q4 and Q5 (it is called active mirror in book) : just testing but not working properly

Using the active mirror, voltage at base of Q1 and Q2 was about 5v...Without the mirror, using current source on Q1, voltage at both bases was about 1V


Thanks
 

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Thread Starter

Zeeus

Joined Apr 17, 2019
540
Hmm...Can not see anything new from the first except the exclamation mark lol..missing something?


Edit : I see :) cap is now connected to output

Thought the cap is to reduce miller effect? (Was about reading miller effect again to understand) but thought, it has to be with the CE transistor?

Please explain miller effect..Thanks
 
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Bordodynov

Joined May 20, 2015
2,600
Take a close look at my first drawing. Especially on the output signal when going through zero. You did not notice the distortion. Unfortunately the input signal masks it. And in the second picture there is no such distortion. And about the Miller effect, this effect is present not only in the transistor, but also in any inverting amplifier. I did not remove this effect.
Such a push-pull repeater class B gives a step. The step size is less if the signal source is higher impedance. In the second embodiment, the input impedance at the input of the output stage is much larger. Apply two of my options on one circuit. Count and you will well see the difference.
 

Thread Starter

Zeeus

Joined Apr 17, 2019
540
Take a close look at my first drawing. Especially on the output signal when going through zero. You did not notice the distortion. Unfortunately the input signal masks it. And in the second picture there is no such distortion. And about the Miller effect, this effect is present not only in the transistor, but also in any inverting amplifier. I did not remove this effect.
Such a push-pull repeater class B gives a step. The step size is less if the signal source is higher impedance. In the second embodiment, the input impedance at the input of the output stage is much larger. Apply two of my options on one circuit. Count and you will well see the difference.
No did not see distortion:) ...So distorting gets reduced by increasing the input impedance (which negative feedback does) or *biasing the push pull*...Thanks.... Missed something?
 

Thread Starter

Zeeus

Joined Apr 17, 2019
540
The cap actually increases the Miller effect to provide a high frequency rolloff compensation.
Please explain the miller effect expert Crutschow...Thanks...All i know is that there's a capacitor between base and collector and with feedback in an inverting amplifier it then increases causing lowpass at base..Why does it cause oscillation?..To reduce miller effect (read from book), add capacitor between base and collector...But two capacitors in parallel add up, no? (Also, why use two different sizes of cap for supply bypass??)
 

crutschow

Joined Mar 14, 2008
24,942
.To reduce miller effect (read from book), add capacitor between base and collector.
That "book" is incorrect.
Please explain the miller effect
From Wikipedia:
the Miller effect accounts for the increase in the equivalent input capacitance of an inverting voltage amplifier due to amplification of the effect of capacitance between the input and output terminals. The virtually increased input capacitance due to the Miller effect is given by

{\displaystyle C_{M}=C(1+A_{v})\,}

where {\displaystyle -A_{v}}
is the voltage gain of the inverting amplifier ({\displaystyle A_{v}}
positive) and {\displaystyle C}
is the feedback capacitance.

So you can see that any added capacitance between the input and the collector, increases the Miller effect.
 
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