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### Author Topic: Help me understand complementary push pull amp  (Read 5882 times)

#### nix85

• Hero Member
• Posts: 1191
##### Re: Help me understand complementary push pull amp
« Reply #15 on: March 10, 2021, 08:52:47 AM »
I got trouble understanding how EXACTLY you imagine half bridge creating AC.

(Let's ignore the fact that you cannot just recreate the missing halfcycle and call it HiFi)

Now you are mentioning resonance and inductors.

Let's first get some basics out of the way...

We all know no electrons pass through a cap, but a changing electric field, displacement current..

We also know reactance of the cap is XC= -1/2πfC

So, higher the frequency and capacitance, more it looks as if cap is not there.

We also know in purely capacitive or inductive circuit voltage and current are 90° out of phase.

Any changing waveform passes "through" a cap, be it pulsed DC or AC.

So halfsinewave passes through a cap as easily as a whole sinewave does.

In other words, when the halfsine ends, with slight delay, voltage would pass through the cap leaving it uncharged.

So let's bring in resonance. If we are talking LC tank instead of a cap alone, then it depends if it's in series of parallel.

Series LC tank at resonance is short circuit, in parallel open circuit..

So, how exactly is other half of the sinewave produced.

#### AlienGrey

• Hero Member
• Posts: 3604
##### Re: Help me understand complementary push pull amp
« Reply #16 on: March 10, 2021, 10:28:06 AM »
It sounds like you dont experiment  a capacitor is an insulator so how can it pass currevt ?
it charges and discharges it's size demnds on its time and frequency and how it's charged

Also if the cap is in series your DC content is lost.

Also if you bias a semiconducter on its on off slope your moving its DC swing in your circuit.

#### nix85

• Hero Member
• Posts: 1191
##### Re: Help me understand complementary push pull amp
« Reply #17 on: March 10, 2021, 11:40:23 AM »
It sounds like you dont experiment  a capacitor is an insulator so how can it pass currevt ?
it charges and discharges it's size demnds on its time and frequency and how it's charged

Also if the cap is in series your DC content is lost.

Also if you bias a semiconducter on its on off slope your moving its DC swing in your circuit.

I do experiment and build but you clearly don't know the basics.

You don't know what displacement current aka capacitor current is, google it.

"Wire has inductance" sooo? Unless it's coiled inductance is negligible.

Everything i wrote is COMMON ELECTRONIC KNOWLEDGE.

Everything you crossed and called wrong is correct.

"It depends on slope bias set up", no it doesn't, you can't recreate the missing halfcycle and call it HiFi.

All changing voltage waveforms indeed pass through a cap, be it pulsed DC or AC... even pulsed DC squarewave signal will pass through a cap at the moment of the change.

Back to school.

#### antijon

• Full Member
• Posts: 230
##### Re: Help me understand complementary push pull amp
« Reply #18 on: March 10, 2021, 03:02:58 PM »
The first image that you posted wouldn't work. As it is nothing would happen on the neg half cycle.

Just saying it's more likely that they forgot to add the label for -VCC at the bottom rail.

Otherwise, you could replace the "ground" at the load with the center tap of a voltage divider. e.g. if you had 12V from VCC rail to bottom rail, when the top transistor closes the load would see 6V+ from VCC to center tap. When the bottom transistor closes the load would see 6V- from center tap to bottom rail. (Don't really know if this would work because I don't do electronics but it seems feasible in my head lol)

#### nix85

• Hero Member
• Posts: 1191
##### Re: Help me understand complementary push pull amp
« Reply #19 on: March 10, 2021, 03:10:07 PM »
They did not forget it is implied and most often symbolized that bottom is also "ground", that is, Vcc-.

Of course "ground" does not always have to be Vcc-, it's just a circuit wide reference point.

Here is the first diagram on google images for push pull amp

Looking at these diagrams one would conclude that during negative halfcycle current flows from ground to ground, lol.

#### nix85

• Hero Member
• Posts: 1191
##### Re: Help me understand complementary push pull amp
« Reply #20 on: March 10, 2021, 05:46:34 PM »
This guy is "explaining" it and of course does not address the issue.

And then we wonder why ppl don't understand shit. Almost nothing is explained right.

#### AlienGrey

• Hero Member
• Posts: 3604
##### Re: Help me understand complementary push pull amp
« Reply #21 on: March 10, 2021, 10:05:18 PM »
If you understand sh would that be a hallo of some sort ?

If you conect the lower transister to a -12 Volts it will go bang !

Do you know why ?

#### antijon

• Full Member
• Posts: 230
##### Re: Help me understand complementary push pull amp
« Reply #22 on: March 10, 2021, 11:45:49 PM »
AlienGrey, is that why they sometimes put a capacitor before the input? I don't know about with a square wave, but a sine would pass through the cap. and show a pos. and neg. value.

Nix, I can see why that would work in the video, at least the diagram with the capacitor before the output load. The cap. charges during the pos. half wave, and negative half wave discharges the cap. through the bottom transistor. The load after the cap. should see a forward and reverse current

#### nix85

• Hero Member
• Posts: 1191
##### Re: Help me understand complementary push pull amp
« Reply #23 on: March 11, 2021, 10:23:19 AM »
Nix, I can see why that would work in the video, at least the diagram with the capacitor before the output load. The cap. charges during the pos. half wave, and negative half wave discharges the cap. through the bottom transistor. The load after the cap. should see a forward and reverse current

Capacitor before the input is to remove any dc.

Yea, that's what i assumed too. Keep in mind

all changing voltage will pass through the cap.

Only the dc bias 1/2 Vcc can remain on it to

discharge in opposite direction.

#### nix85

• Hero Member
• Posts: 1191
##### Re: Help me understand complementary push pull amp
« Reply #24 on: March 11, 2021, 10:24:45 AM »
If you understand sh would that be a hallo of some sort ?

If you conect the lower transister to a -12 Volts it will go bang !

Do you know why ?

LOL i see you are still confused,
have you at least learned what displacement current is
and that all changing signals pass through a cap ?

#### nix85

• Hero Member
• Posts: 1191
##### Re: Help me understand complementary push pull amp
« Reply #25 on: March 11, 2021, 11:16:30 AM »
Only way i can see it really make sense, the way he describes in the video

is with another resistor that forms voltage divider with load resistor

that biases Ve at 1/2 Vcc when both transistors are off. So as Q1 turns on Ve

can swing from 1/2 Vcc up to Vcc and when Q1 is off and Q2 on from 1/2 Vcc to 0.

#### nix85

• Hero Member
• Posts: 1191
##### Re: Help me understand complementary push pull amp
« Reply #26 on: November 09, 2021, 03:07:46 PM »