# Homework Help.... Design of transformer coupled Push Pull (Class-B)

#### asbjc

Joined Oct 30, 2020
32
Hi !

I'm just a beginner with EE. And i got a problem to solve.

" Design of a transformer coupled (Input+ Output) push pull (class B) stage using studied NPN and PNP for the load current of (5)Amps. Also make a comparison of efficiency with that of the direct coupled amplifier. "

Does any one know whats the simple meaning of this problem. Especially in the following aspects:

Transformer coupled input and output ?
What would be the difference in output of transformer coupled & direct coupled amplifier ?

Thanks for your interest in helping me out with my homework....

#### ericgibbs

Joined Jan 29, 2010
11,551
hi asbjc

Welcome to AAC.

E

#### Ramussons

Joined May 3, 2013
911
Transformer coupled input and output ?
The output stage is a Class B setup with a Transformer Output.
The output stage is driven through a Transformer.
Hint: Google Transformer coupled Class B amplifier.

#### Audioguru again

Joined Oct 21, 2019
2,353
Transformers in amplifiers were used a LONG time ago. Not anymore.
Beware that many Google circuits show a transformer coupled class-AB amplifier, which is different to a class-B distorted amplifier.
Why don't you have a teacher and have a text book??

#### asbjc

Joined Oct 30, 2020
32
hi asbjc

Welcome to AAC.

E
Hi !

I'm done with the first portion of designing the Push-Pull (Class-B) stage amplifier for 5 Amps output.
(from input 5v @ 1khz, the simulated output across the load was 5v @ 1khz at 5amps)

Does this design look good ?

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#### Ian0

Joined Aug 7, 2020
1,101
I hope you don't have to build it - no one has made transformers for transistor amplifier output stages in the last forty years!

#### Audioguru again

Joined Oct 21, 2019
2,353
I'm done with the first portion of designing the Push-Pull (Class-B) stage amplifier for 5 Amps output.
(from input 5v @ 1khz, the simulated output across the load was 5v @ 1khz at 5amps)
Does this design look good ?
The output is 5V peak into 1 ohm which is 25W peak. Why not use a real speaker that is NEVER 1 ohm? Most home speakers are 8 ohms and car speakers are usually 4 ohms.
You have a 20V supply so for 25W peak, each transistor drives 15 ohms as you show, then the current in each transistor is 1.29A peak.

Why didn't you or your simulation software look at the datasheets of the low power little 2N3904 and 2N3906? Their absolute maximum current is only 200mA and they work poorly above about 100mA.

Without biasing the transistors into class-AB, the crossover distortion will sound horrible.

#### KeithWalker

Joined Jul 10, 2017
1,386
The load current is supposed to be 5A. Did they specify whether that is 5A RMS, peak or peak to peak? If it was not specified, you should assume that they meant 5A RMS and state that in your solution. You should also make sure that your solution can deliver 5A RMS. To do this, you need to specify the turns ratio of the primary to secondary on both transformers.

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#### Audioguru again

Joined Oct 21, 2019
2,353
He is not making the very old amplifier design, instead he is simulating it and the voltages and currents in the simulation are all peak.
The student, teacher and simulator do not know that speakers are 4 or 8 ohms and that little 2N3904 and 2N3906 transistors will release their magic smoke with a 5A current.

#### asbjc

Joined Oct 30, 2020
32
He is not making the very old amplifier design, instead he is simulating it and the voltages and currents in the simulation are all peak.
The student, teacher and simulator do not know that speakers are 4 or 8 ohms and that little 2N3904 and 2N3906 transistors will release their magic smoke with a 5A current.
Thanks, i got it.
I've made some changes. (Used TIP31, 32 as NPN, PNP for new output of 1A. TIP31 & 32 satisfy this current rating. & this time a 5ohm speaker).
Is it fine now ?

#### asbjc

Joined Oct 30, 2020
32
Thanks, i got it.
I've made some changes. (Used TIP31, 32 as NPN, PNP for new output of 1A. TIP31 & 32 satisfy this current rating. & this time a 5ohm speaker).
Is it fine now ?

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#### Ian0

Joined Aug 7, 2020
1,101
And how well does it work?

#### asbjc

Joined Oct 30, 2020
32
And how well does it work?
Simulation gives right 1A across the load.
Looks fine to me.

I'm just little confused, how different it is from #6 circuit.
Mine contains transformer coming from Emitter. While #6 and mostly on internet, collector terminals are used for transformer. Just need a little explanation here.

#### Ian0

Joined Aug 7, 2020
1,101
How much does the output waveform resemble a sinewave?

The most important difference is the Post #6 circuit uses two NPN transistors, and transformers with three windings. One primary and two secondaries for the drive transformer, and two primaries and one secondary for the output transformer.
That reverses the polarity of the signal being handled by one of the transistors. So, both transistors are dealing with signals that are positive, so they can both be NPN, and a negative supply is not needed.

The other circuit uses a PNP transistor and a negative supply to handle the negative part of the signal.
Run the simulation and you will see.

Just for interest, the post #6 circuit is much like a thermionic valve output stage, as valves can only exist as "N-channel".

#### asbjc

Joined Oct 30, 2020
32
How much does the output waveform resemble a sinewave?

The most important difference is the Post #6 circuit uses two NPN transistors, and transformers with three windings. One primary and two secondaries for the drive transformer, and two primaries and one secondary for the output transformer.
That reverses the polarity of the signal being handled by one of the transistors. So, both transistors are dealing with signals that are positive, so they can both be NPN, and a negative supply is not needed.

The other circuit uses a PNP transistor and a negative supply to handle the negative part of the signal.
Run the simulation and you will see.

Just for interest, the post #6 circuit is much like a thermionic valve output stage, as valves can only exist as "N-channel".

Output:

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#### asbjc

Joined Oct 30, 2020
32
How much does the output waveform resemble a sinewave?

The most important difference is the Post #6 circuit uses two NPN transistors, and transformers with three windings. One primary and two secondaries for the drive transformer, and two primaries and one secondary for the output transformer.
That reverses the polarity of the signal being handled by one of the transistors. So, both transistors are dealing with signals that are positive, so they can both be NPN, and a negative supply is not needed.

The other circuit uses a PNP transistor and a negative supply to handle the negative part of the signal.
Run the simulation and you will see.

Just for interest, the post #6 circuit is much like a thermionic valve output stage, as valves can only exist as "N-channel".
Thanks a lot for the explanation. I got your point.

So, now in the second part of Qs, i'm asked,

>> Make a comparison of its efficiency with that of the direct coupled amplifier.

& i'm new to this "Direct Coupled using TIP31, TIP32" thing. I've tried to find any resources on internet, but differences were confusing. Is there any recommendable good article or video description to this topic ?

#### Ian0

Joined Aug 7, 2020
1,101
Not a bad sinewave! You'd not sell it as hi-fi these days, but back in the 1960s you might have been able to.
This is about the simplest "direct cooupled" amplifier I could find.
http://amplifiercircuit.net/easy-100-watts-power-amplifier.html
For low power, the output transistors could be TIP31/32 instead of TIP142/147.
In order to answer the question - what is there in your circuit that isn't in this one? And how efficient is that component?

#### Ramussons

Joined May 3, 2013
911
Not a bad sinewave! You'd not sell it as hi-fi these days, but back in the 1960s you might have been able to.
This is about the simplest "direct cooupled" amplifier I could find.
http://amplifiercircuit.net/easy-100-watts-power-amplifier.html
For low power, the output transistors could be TIP31/32 instead of TIP142/147.
In order to answer the question - what is there in your circuit that isn't in this one? And how efficient is that component?
Is'nt this Class AB? D1 and D2 needs to be bypassed for Class B.

#### Ian0

Joined Aug 7, 2020
1,101
Forgot that requirement! But two diodes minus two Vbe junctions is pretty close to Class B. It will depend on the choice of diode.
With the diodes shorted out it would be class C.
Two 1N4148s and it might be slightly in Class AB, two 1N4001s would make it damn close to Class B.
I suppose that to make it perfectly Class B, then it should have two more TIP31/32s with their collectors open or connected to their bases as the diodes, and they should be firmly fastened to the same heatsink as the output transistors.

(As the original had darlingtons, it would be severely underbiassed)

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