About a capacitor between the drivers in somes amp

[gaetan8888]

Hi

What is the use of the capacitor C7 between the driver of some power amp ?



Thank

Gaetan

[Twaksak]

The purpose of your r17 / c3 components is to avoid cross conduction commonly called switching distortion due to sluggishness caused by capacitance in output stage power transistors. If you assume that a positive voltage swing has just occured leaving a positive charge at the base of Q2. As the voltage from the VA swings negative, the voltage at the emitter of Q8 must already be 0.67 volt more negative than the emmitter of Q3 in order to overcome Q3's Vbe drop. As current begins to flow through R5 , it also begins to drop a negative voltage, which is corrected at Q8's emitter, increasing the negative potential. This small negative potential is applied to the base of Q2 through R27 and C7 (speed up capacitor). The negative potential forces the positive charge carriers to be rapidly sucked out of Q2's base and results in rapid turn off. The opposite is true for the next cycle.

[richie00boy]

Yes, it's commonly called a 'speed-up capacitor'.

[pooge]

See here:

http://users.ece.gatech.edu/~mleach/lowtim/output.html


and here:

http://www.diyaudio.com/forums/showt...303#post406303

[gaetan8888]

Hello

Do a 1 uf value are standard or it's need to be calculated for each kind of power amp ?

Thank

Gaetan

[richie00boy]

It does need to be determined experimentally really, but some do get away with guessing.

[gaetan8888]

Originally posted by richie00boy
It does need to be determined experimentally really, but some do get away with guessing.

Hi

Is there a simple method to determine it experimentally ?

Thank

Gaetan

[richie00boy]

Oscilloscope, square waves, and ammeter in the supply.

[analog_guy]

Actually, the purpose of the capacitor C7 is to guarantee that the voltage between the bases of the positive and negative output devices remains constant. Most amplifiers employ some type of output biasing circuit to minimize crossover distortion where neither the positive nor negative device would otherwise be in conduction. The biasing voltage needs to remain at a fixed voltage independent of the voltage swing. C7 serves the purpose of holding that bias voltage constant.

This particular amplifier example probably uses a constant current source to drive bias resistor and therefore to generate a fixed bias voltage. Another popular circuit is the Vbe multiplier which consists of a voltage divider driving the base of a BJT. In this case it is also usual to include a 0.1 - 1.0 uf capacitor between the emitter and collector of the Vbe multiplier transistor.

[vynuhl.addict]

Analog Guy,


I think you may have this cap mixed up with a VBE multiplier bypass cap in which you speak of, in that case it helps regulate voltage and benefits the VBE transistors stability. In the case of C7 it is a charge suckout cap helpiing speedup the switching off of the output bases in a Class A/B amplfier. There are those who say this doesnt make a difference with +/- rails, namely Leach, but i have found it personally to make a big difference sonically.


Colin