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Diogo 23rd October 2002 07:35 PM

Symmetrical 400W amplifier
A newbie question, In this amplifier I do not understand how Q4, Q5, ZD2, R3, R4, R5 works, I believe it's a current sink but I'm not sure how it works, would like some matematical explanations.

The voltage gain stage used is a differential pair right?

Thanks for any help

peranders 23rd October 2002 08:42 PM

hmm..what amp are talking about? :confused:

Diogo 24th October 2002 01:49 AM

peranders 24th October 2002 09:01 AM

Q4 and Q5 forms a cascode. You can read more about it at... can't reach at the moment.

The purpose is:

* Make the input stage faster (5-10 times!)

* Withstand higher voltage

Q4 and Q5 are base grounded stages. The collector load of Q2 is more or less zero (diod plus a zener down to ground = low impedance) therefore the "Miller" effect is small. This effect slows down the transistor.

The cascode has no effect of gain nor input noise. This two transistor circuit forms a "super transistor", fast, low noise, high voltage, high Hfe (usually)

traderbam 24th October 2002 09:33 AM

In other words, Q4 and Q5 just regulate the voltage at Q2 and Q3 collectors to stop their Vce rating being exceeded - as the psu voltage is 70V (BC549 are rated at 65V). The current from Q2 and Q3 just passes straight through Q4 and Q5.

I don't think the speed of the input stage is much affected by Q4 and Q5 because the collector loads of 2.2k are small, so the miller effect is not so important. In fact, usually you don't want the input stage too be too fast because it affects the stability of the amp. However, you do get capacitive coupling across the be of the transistors which has not been decoupled to ground in this design. For better performance I'd consider removing Q4 and Q5 and replacing them with two zeners.

Diogo 24th October 2002 11:17 AM

Ok I think I undertand Why and how, but I still whant to know how to calculate this cascode stage voltage and currents, the resistor values and the zener voltage.

Thank you guys!

peranders 24th October 2002 05:53 PM

I forgot one thing about the cascodes, they produces less distortion, not a bad thing. :)

peranders 24th October 2002 05:58 PM


Originally posted by traderbam
In fact, usually you don't want the input stage too be too fast because it affects the stability of the amp.
My point of view is quite the opposite! What a fast input stage you will only get one dominant pole = easier to feedback => better stability.

This is especially true if you want to make a high slew rate amp. The other poles are far far away and can't make no harm.

traderbam 24th October 2002 06:25 PM

"This is especially true if you want to make a high slew rate amp"
Why do you want a high slew rate amp? The highest frequency being amplified is 20kHz isn't it? What slew rate do you consider acceptable?

peranders 24th October 2002 07:44 PM

Slow amps tends to have high distortion in the higher frequencies.

I think it's wise make an amp with a power bandwidth of > 100 kHz but > 300-400 kHz aren't neccessary.

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