Question about Class B audio amplifier

Thread Starter

simpsonss

Joined Jul 8, 2008
173
Push goes negative and pull goes positive. They cancel at the output.
sorry, not really understand what u mean.Can u explain a bit more? thanks.
As what i know, when the output of Op-amp goes positive, Q3 will in active mode and Q4 is conducting(which means push happen). Am i correct at this point?

thank you.
 
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Thread Starter

simpsonss

Joined Jul 8, 2008
173
sorry, not really understand what u mean.Can u explain a bit more? thanks.
As what i know, when the output of Op-amp goes positive, Q3 will in active mode and Q4 is conducting(which means push happen). Am i correct at this point?

thank you.
How about this?thx.
 

Thread Starter

simpsonss

Joined Jul 8, 2008
173
hi,

push pull.JPG
Weird1.jpg

I get the output waveform as the picture above which the positive sine wave looks like a bit weird. what usually cause this to happen?

thank you.
 

Audioguru

Joined Dec 20, 2007
11,248
If the sine-wave at the top is the input from a signal generator then you can see that its lower portion is oscillating at a very high frequency.
The output of the amplifier shows the top half with the very high frequency oscillation from the signal generator amplified. The amplifier inverts the signal.

Usually high frequency oscillation occurs because an amplifier feeds a long wire or shielded cable.
Using a breadboard instead of a compact pcb also causes an amplifier circuit to oscillate at a very high frequency.
Most amplifier circuits use a Zobel Network (look in Google) to be a load on the amplifier at very high frequencies where the reactance of the inductance of a speaker causes its impedance to be high.
 

Audioguru

Joined Dec 20, 2007
11,248
1) First you need to make your signal generator produce a smooth sine-wave without the very high frequency oscillation.
2) Make the circuit on a compact pcb if it is now on a breadboard.
3) Then you need to add 0.1uF ceramic bypass capacitors with short leads at the positive and negative supplies for the emitter-follower output transistors.
4) Then add a Zobel network at the output (0.1uF ceramic capacitor in series with 10 ohms to ground).
 

Thread Starter

simpsonss

Joined Jul 8, 2008
173
1)
3) Then you need to add 0.1uF ceramic bypass capacitors with short leads at the positive and negative supplies for the emitter-follower output transistors.
yea, just added in the zobel networks.the high frequency oscillation already eliminated.it works.thanks guru.

thanks,
 
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Thread Starter

simpsonss

Joined Jul 8, 2008
173
what i know about crossover distortion when trying at some simple amplifier is when i tune the bias to high and it cause the output to crossover distortion.

ok.will google it.
 

Audioguru

Joined Dec 20, 2007
11,248
Your circuit has high crossover distortion because the bases of the output transistors are connected together and operate in class-B. Without a signal or with a low level signal both output transistors are turned off and do not follow the input. The output transistors do nothing until the input is 1.3V p-p and signals with less amplitude produce severe crossover distortion.

If two forward-biased diodes are connected between the bases of the output transistors then the transistors will be turned on slightly and the crossover distortion will be very low. Diodes are used because they have the same voltage as the base-emitter diodes of the transistors and if they are bolted to the heatsink of the output transistors then the diodes change their voltage exactly the same as the base-emitter voltage of the transistors change when the temperature changes.
If the feedback to the opamp includes the output transistors then the crossover distortion can be reduced more.

EDIT:
Many amplifiers use an adjustable transistor instead of two diodes. It is called a Vbe multiplier.
The transistor is bolted to the heatsink of the output transistors and its voltage tracks the voltage of the base-emitter of both transistors as the voltage changes with temperature change.
The adjustment of the idle current is possible with a trimpot.
 
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Thread Starter

simpsonss

Joined Jul 8, 2008
173
yeap. after follow the tutorial of the worksheet, i have added in two diodes to make the two transistor turn ON at the crossover distortion zone and this make the circuit operate in class-AB.

Then the circuit is modified again as figure below.
untitled.JPG

i haven really figure out how this final circuits work. but it seems like still have a very very very little crossover distortion appear.

thank again for all the guidance,
 

Audioguru

Joined Dec 20, 2007
11,248
I see only one diode in your circuit. It needs two diodes because the transistors have two base-emitter junctions. You also added negative feedback from the output to the second opamp that helps reduce distortion.

Your circuit has a problem because the 10k resistor cannot supply 40mA or more to the base of the TIP42 transistor to turn it on completely. Also the second opamp cannot provide 40mA or more to the TIP41 plus more current for the 10k resistor.

The output transistors must be darlingtons or have driver transistors for enough current gain.
 

Thread Starter

simpsonss

Joined Jul 8, 2008
173
at the worksheet, there mentioned that for the circuit that using two diodes:

This solution doesn't prevent simultaneous turn-on of the two transistors, but merely reduces the severity of the problem and prevents thermal runaway. It also has the unfortunate effect of inserting resistance in the load current path, limiting the output current of the amplifier.
---------------------------------------------------------------------------------------

Then, the author introduced the final circuit which is as shown in post #55,which he mentioned that,

I use one diode to provide a 0.7 volt bias voltage for the push-pull pair. This is not enough to eliminate the "dead" signal zone, but it reduces it by at least 50%
-----------------------------------------------------------
Since the voltage drop of a single diode will always be less than the combined voltage drops of the two transistors' base-emitter junctions, the transistors can never turn on simultaneously, thereby preventing class AB operation. Next, to help get rid of the remaining crossover distortion, the feedback signal of the op-amp is taken from the output terminal of the amplifier
these are what the author mentioned.
 

Audioguru

Joined Dec 20, 2007
11,248
High fidelity amplifiers operate in class-AB so that they do not have any crossover distortion and do not get hot.
Your amplifier has only a single diode so its output transistors operate in class-B. Since the opamp does not have enough gain at high frequencies to cancel it then the crossover distortion will be audible.
 

Thread Starter

simpsonss

Joined Jul 8, 2008
173
ok guru, i understand what u mean.

ok. let me try to construct a class AB pushpull using two diodes. Below is the circuit from the worksheet too. But i got no component value for the resistor and the diode ? Does diode 1n4148 suitable? what value should be for the four resistors?

pushpull.JPG

thanks.
 
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