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Discussion Starter · #1 ·
Hi everyone,

I've been searching a lot for this, but haven't found a satisfying answer. My question is : how come do the sine waves don't cancel each other in a push-pull class A amp, since the transistor (or valve) works in the opposite phase as the other, while not shutting off like in a class B?

Thank you,
Benjamin
 

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Handwaving explanation: Class A just means 100 % bias all the time so both devices are on all the time. 'ish. Complementary devices (or not, i.e. when tubes and a transformer are involved to do polarity inversion) still work together and, depending upon the design, may be driven together (in phase) but the output of one device draws from the (+) rail and the other from the (-) rail. The circuits (schematic) may look the same for class A, AB, and B (F/G/H for that matter) with the only difference the amount of bias current flowing. When the signal gets out of range of one device, it quits contributing to output current, but the bias current in the device is still flowing (at 100 %). The big advantage is that efficiency for a push-pull (complementary) class A design rises to ~50 % theoretical maximum up from the 27 % or so max of a single-ended class-A design.

A tube version: http://www.aikenamps.com/index.php/what-do-the-terms-push-pull-and-single-ended-mean

Bottom line: at the output the devices' outputs do not cancel; if they invert phase, the input to them is inverted so they all work together at the output.

Douglas Self has some great books on amplifier design. Horwitz' Art of Electronics is a good reference.

HTH - Don
 

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Discussion Starter · #3 ·
Thank you for the reply, helps me understand a bit more about the technical aspects of amplifiers! This makes me wonder : does the transistor that receives the inverted signal need to be inverted again before the output in order to complete the sine wave (so it does not only output positive halves)? Also, does a transistor (or valve) stop sending signal when the voltage falls below zero and that's why even if it's turned ON it will not send signal or it's another phenomenon that happens?

P.S. Sorry for the number of questions, it's really a question that bugs me a lot and I don't quite get it completely!
 

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You should become an EE!

Depending upon the topology the transistor may invert polarity, or not. Yes at the output everything has to be in phase to produce an output.

The answer to the second question is a bit tricky. A device, tube or transistor, turns off when the applied control voltage drops below its turn-on (threshold) voltage. It is not like a switch, the device turns off more gradually than that. The control voltage is across a couple of terminals of the device, e.g. the base and emitter for a BJT, gate and source for a FET, gate and cathode for a tube. The output device of a class A circuit is often a simple voltage follower and is always on, with essentially constant voltage across the two control terminals. The output simply follows the input voltage. In a push-pull design, one device conducts current in one direction, and the other in the opposite direction. As the input voltage goes up and down the outputs follow, but at some point the current output (drawn by the load, a speaker in this case) changes polarity and so one device does not supply any current to the load. It is still on, but the current through it does not go to the output. And similar for the other (complementary) device.

Wikipedia or one of the aforementioned texts would help. As would a whiteboard and face-to-face. It takes too much work to type and is much easier to explain with pictures. You might try a Google search on amplifiers and amplifier classes of operation.
 

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Discussion Starter · #5 ·
If EE means Electrical Engineer, it's a bit too late. I've already done a semester in mechanical engineering but switched to business, so I don't really see myself going back again to engineering! However, thank you very much for your good answers, very much appreciated and I'm still interested in learning about engineering in audio (and cars)!

Thank you,
Benjamin
 

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Hi everyone,

I've been searching a lot for this, but haven't found a satisfying answer. My question is : how come do the sine waves don't cancel each other in a push-pull class A amp, since the transistor (or valve) works in the opposite phase as the other, while not shutting off like in a class B?

Thank you,
Benjamin
Let's build a Class A Push Pull Amp:

Imagine a pair of Batteries, with two series resistors, and the speaker attached between the resistors, capacitively coupled.



When resistors A and B are of equal resistance, the point between the two resistors is at ground potential.

If resistor A becomes less resistive, and B becomes more resistive, then the output moves toward Positive, and current flows through the speaker toward the positive terminal.

If resistor A becomes more resistive, and B becomes less resistive, then the output moves toward Negative, and current flows through the speaker toward its ground.

---

So, the change in the resistors are 'out of phase'.

Replace the resistors with transistors. The transistors are variable resistors.

You want A turning more 'on' (less resistive) when B is turning more 'off' (more resistive), and A turning more 'off' when B is turning more 'on', so, the signals (music to be amplified) applied to control the transistors are 'out of phase'.

---

If you don't understand voltage drop across series resistances, go to electronics class for at least the first day of lab.
 

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Let's build a Class A Push Pull Amp:

Imagine a pair of Batteries, with two series resistors, and the speaker attached between the resistors, capacitively coupled.



When resistors A and B are of equal resistance, the point between the two resistors is at ground potential.

If resistor A becomes less resistive, and B becomes more resistive, then the output moves toward Positive, and current flows through the speaker toward the positive terminal.

If resistor A becomes more resistive, and B becomes less resistive, then the output moves toward Negative, and current flows through the speaker toward its ground.

---

So, the change in the resistors are 'out of phase'.

Replace the resistors with transistors. The transistors are variable resistors.

You want A turning more 'on' (less resistive) when B is turning more 'off' (more resistive), and A turning more 'off' when B is turning more 'on', so, the signals (music to be amplified) applied to control the transistors are 'out of phase'.

---

If you don't understand voltage drop across series resistances, go to electronics class for at least the first day of lab.
Best explanation of a Push pull class A amp I've ever read! Kudos!

In what sense does the class B amps differ from that?
 

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Best explanation of a Push pull class A amp I've ever read! Kudos!

In what sense does the class B amps differ from that?
Class B - the transistors would be operated so that only one transistor is turned on at a time while the signal goes postive and negative.

Positive input turns leaves the B transistor turned off, turns on the A transistor, and current flows from ground through the speaker toward the positive terminal of the battery.

Negative input turns leaves the A transistor turned off, turns on the B transistor, and current flows from the negative battery through the speaker to ground.

With no signal, both transistors would be turned off.
 

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Wouldn't it need a wave generator and a way of modulating the wave to be part of class d?
Lots of parts are missing, to make it ANY kind of amplifier.
 
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