# I need to simulate a class B push pull amplifier with center tapped transformers to get 1A load current.

#### Mustaqeem Tariq

Joined Oct 30, 2019
21
The load could be of any resistance. I need some help in making this circuit in LTspice as I am trying this for the first time. I only know the basics. Any help would be appreciated. The voltage at the secondary winding of input transformer is not the same as the input at the primary windings but in theory it should be same.

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#### Audioguru again

Joined Oct 21, 2019
6,691
Transformers were used 60 years ago.
Your transistors are not biased so they produce severe distortion. They do nothing until the base voltage becomes about 0.7V then they amplify. Here is an old circuit:

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#### MisterBill2

Joined Jan 23, 2018
18,504
That circuit can amplify, but a.g. is half right in that it will have a fair amount of distortion. What does not look right to me is the inductance values selected for the transformer windings. With a 16 ohm load I am guessing that it is an audio amplifier. What impedance have you selected, and more important, what ratio did you choose? 1:1 can work, but to get 1 amp into 16 ohms you will need 16 volts. So the transistors will need to be driven close to saturation with a 1:1 ratio of one half the primary to the secondary. Assuming no losses.
But this is not the normal way to design amplifiers.

#### Ylli

Joined Nov 13, 2015
1,087
The inductance values looked a bit low to me too. The transformer turns ratio will be the √ of the inductance ratio, so these are 5:1 transformers with the '1' side center tapped.

#### Mustaqeem Tariq

Joined Oct 30, 2019
21
I modified the circuit design but it still does not give 1A current at the load. Please someone guide me. I am completely new at this thing. The modified circuit is as follows:

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#### Wolframore

Joined Jan 21, 2019
2,610
I thought the assignment is to build a class B.

#### Mustaqeem Tariq

Joined Oct 30, 2019
21
Yes I have to simulate a class B push pull amplifier. I used this diagram as reference:

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#### ronsimpson

Joined Oct 7, 2019
3,037
You can not get 1A in the transistors. Can you see why? What limits the current?

#### Mustaqeem Tariq

Joined Oct 30, 2019
21
I know that I1 - I2 will be there on the load but it should be zero as both the currents are approximately same. But in my project it is mentioned that:
• Design and Implementation of a transformer coupled (at input and output) push pull (class B) stage using TIP31 and TIP32 for a load current of 1A and compare its efficiency with that of the direct coupled amplifier.

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#### ronsimpson

Joined Oct 7, 2019
3,037
1) 2N4124 is only a 0.2A transistor. So I changed to 2SCR574D. 2A transistor.
2) The signal is 1khz (audio) but the transformer is for Radio Frequencies. The transformer is shorting out the signal. Either change the frequency to RF or change to a audio transformer. (make the inductance much larger) Right now the signal source is pushing 15A into the transformer. I changed the inductance from uH to mH. (inductors short out low frequencies)
3) The transistor current is set by the emitter resistors. 100 ohms 1A said you need 100V of drive into the Base. Changed to 2.2 ohms to get the current up.
4) 2.2k resistor to supply current for the Base of the transistors. Not enough current changed to 1k. Probably should be 470 ohms.
5) I changed the turn ratio on the input transformer.
Now the output is 9Vpk or 18Vp-p and slightly over 1A.
I know this is home work and you need to know why each change. But there was so many problems that fixing one at a time will take all day. So go through the change and learn why. Ask questions.

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#### Mustaqeem Tariq

Joined Oct 30, 2019
21
Thanks man. You saved me from getting zero marks. Can we implement this circuit using TIP31 transistor. Anyway thanks for the help.

#### MisterBill2

Joined Jan 23, 2018
18,504
TIP31 (NPN) and TIP32 (PNP) are a complementary transistors, and would not be used in a normal push-pull power stage. So while the circuit posted is OK, there is a bit of confusion on my part about what transistors would be used, what the assigned transistors were, and how the circuit is expected to be designed.
And the class "B" operation means that it will be rather critical as to where you set the base bias and the no-signal current.

#### Mustaqeem Tariq

Joined Oct 30, 2019
21
The transistor assigned to us was TIP 31. The question is as follows:
• Design and Implementation of a transformer coupled (at input and output) push pull (class B) stage using TIP31 and TIP32 for a load current of 1A and compare its efficiency with that of the direct coupled amplifier.

#### MisterBill2

Joined Jan 23, 2018
18,504
If the circuit MUST use both the TIP31 AND a TIP32, then it will be different from the one in post #10. That is a good circuit BUT TIP31 (NPN) and TIP32 (PNP) are a complementary pair transistors, and that makes the circuit quite different. The early application of complementary pair transistors was to avoid the need for an output transformer. So suddenly the task is quite different, if both transistors are to be used.

Did you already study a circuit for a direct coupled amplifier for this application? There is a whole lot of background information missing here. I did not initially realize that this was a take-home final exam.

#### Mustaqeem Tariq

Joined Oct 30, 2019
21
Yes I studied the direct coupled amplifier but there is no transformer involved in the complementary pair transistor. I think that the task is to implement the circuit using either of the TIP31 or TIP32 transistor.

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#### MisterBill2

Joined Jan 23, 2018
18,504
Yes I studied the direct coupled amplifier but there is no transformer involved in the complementary pair transistor. I think that the task is to implement the circuit using either of the TIP31 or TIP32 transistor.
The circuit posted here would use both TIP31 and TIP32. It is definitely a complementary amplifier. Adding a transformer to the output in place of the load resistor shown could improve the efficiency by providing optimum matching of the load resistance to the amplifier output impedance. Transformer losses would reduce the efficiency by the power dissipated in the transformer, probably the losses will be less than the increases due to better matching.
On the input side there will be an efficiency improvement through better matching the drive source impedence to the base impedance. BUT the input transformer will need a center tap on the base winding to the junction of the 2 emitters. AND you may need to add some bias circuit to get to class B operation.

#### Audioguru again

Joined Oct 21, 2019
6,691
Audio amplifiers are never "class-B" that produces awful crossover distortion, instead they are class-AB or today many audio amplifiers are class-D.
Modern audio amplifiers never use an output transformer unless it is to match 70V transformers on many speakers in a store.
60 years ago amplifiers with vacuum tubes used an output transformer to match the high impedance tubes to a low impedance speaker.

A modern audio amplifier never "matches" the impedance of a speaker for better efficiency, instead it has a very low output impedance for good damping of speaker resonances.

#### Mustaqeem Tariq

Joined Oct 30, 2019
21
The circuit posted here would use both TIP31 and TIP32. It is definitely a complementary amplifier. Adding a transformer to the output in place of the load resistor shown could improve the efficiency by providing optimum matching of the load resistance to the amplifier output impedance. Transformer losses would reduce the efficiency by the power dissipated in the transformer, probably the losses will be less than the increases due to better matching.
On the input side there will be an efficiency improvement through better matching the drive source impedence to the base impedance. BUT the input transformer will need a center tap on the base winding to the junction of the 2 emitters. AND you may need to add some bias circuit to get to class B operation.
Can you please explain the bias circuit to get to class B operation?

#### MisterBill2

Joined Jan 23, 2018
18,504
This system is a textbook project and probably does not connect to a speaker, since that is a complex inductive load. And the instructions say "Class B", and they do not mention distortion, only efficiency. Efficiency is seldom the primary goal of low distortion amplifiers, I am sure you will agree with that.

#### Mustaqeem Tariq

Joined Oct 30, 2019
21
This system is a textbook project and probably does not connect to a speaker, since that is a complex inductive load. And the instructions say "Class B", and they do not mention distortion, only efficiency. Efficiency is seldom the primary goal of low distortion amplifiers, I am sure you will agree with that.
Yes, you are right