# Class B -Transformer Coupled Push Pull Amplifier

#### Vckay

Joined Aug 7, 2005
14
Ok.. Here is the problem. I was studying about the transformer coupled classs B push pull amplifier ( using 2 npn transistors ).

The input signal is connected to a transformer which produces 2 out of phase signals ( centre tapping provided). Each of these 180 out of phase signals are supplied each to a npn transistor.
This means that Transistor 1 conducts during the first half cycle ( when input to it is positive and input to second transistor is negative ) Transistor 2 conducts during the second half cycle ( when input to is positive and input to first transistor is negative).
The output of both transistors go to an output transformer whose centre tap is connected to +Vcc. And a sinusoidal output is obtained..
Here comes my confusion...
My textbook ( Electronic devices by floyd) shows that first half cycle by T1 and undergoes a 180 degree phase shift and comes out as a negative half cycle... Similarily the second half cycle by T2 undergoes a 180 degree phase shift and comes out as a negative half cycle...
But 2 negative half cycles given as input to a centre tapped transformer CANT surely bring about a correct sinusoidal waveform as output ...Can they????

P.S if nobody has understood my doubt , i will try to upload a picture next time to make it clearer.

#### Erin G.

Joined Mar 3, 2005
167
Post the picture if you can. It would be a big help in our understanding of what you're looking at.

#### nomurphy

Joined Aug 8, 2005
567
Sure, it just depends on the connection polarity to the transformers.

#### Vckay

Joined Aug 7, 2005
14
Originally posted by JoeJester@Aug 31 2005, 08:12 AM
Vckay,

Try the attached diagram. It agrees with your text.
[post=10047]Quoted post[/post]​

Joe
Thank you very much for the picture. Now I will be able to explain my problem better.
The output of each transistor is a negative half cycle . What i don't get is how come the final output from the transformer is a sinusoidal wave..
Cause the 2 half cycles of the same polarity are similar to the output from a FWB (only in opposite polarity) . Isn't that like rippled DC?

Thanks again for all your help, People.

#### Haus76

Joined Aug 31, 2005
7
Originally posted by Vckay@Sep 1 2005, 01:47 AM
Joe
Thank you very much for the picture. Now I will be able to explain my problem better.
The output of each transistor is a negative half cycle . What i don't get is how come the final output from the transformer is a sinusoidal wave..
Cause the 2 half cycles of the same polarity are similar to the output from a FWB (only in opposite polarity) . Isn't that like rippled DC?

Thanks again for all your help, People.
[post=10069]Quoted post[/post]​

The sinusoidal wave shape is due to the polarity of the current through the second transformer. As T1 turns on, current flow produces the positive going component. When T2 is on, the current flow is in the opposite direction causing the second half of the waveform. Just remember that the entire wave form has positive voltage.

#### Haus76

Joined Aug 31, 2005
7
Originally posted by Haus76@Sep 1 2005, 03:34 PM
The sinusoidal wave shape is due to the polarity of the current through the second transformer. As T1 turns on, current flow produces the positive going component. When T2 is on, the current flow is in the opposite direction causing the second half of the waveform. Just remember that the entire wave form has positive voltage.
[post=10076]Quoted post[/post]​

Correction: I forgot that the output transformer was center-tapped on the primary...sorry

#### JoeJester

Joined Apr 26, 2005
4,390
Vckay,

It's not like ripple. Ripple is related to the line frequency of the line input to the power supply ... i.e. 60 Hz, 120 Hz, 240 Hz ... etc. Ripple is sychronized to the incoming mains.

The signal you seen at the output of the the transistors is the voltage difference when a signal is applied.

Just as the input transformer takes a single phase signal and produces two out of phase signals [referenced to the center tap], the output transformer does the reverse ... combines the two signals.

Look at the position of the outputs relative to the input signal's phase.

Class B operation is typically for one half cycle.

#### Rick Martin

Joined Jun 14, 2009
31
Vckay,

Try the attached diagram. It agrees with your text.
Sorry for dragging up an extremely old thread but I have the same question as above from the same text, you wouldnt happen to have the attached diagram handy?