Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronic Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

Transformer coupled Class A amplifier

paulmdrdo

Full Member level 3
Joined
Jan 17, 2014
Messages
175
Helped
1
Reputation
2
Reaction score
1
Trophy points
18
Activity points
1,365
Hi Guys! I'm having some kind of difficulty understanding how the voltage VCE of a transformer coupled class A amp can theoretically swing from 0V to 2Vcc. In class I learned that it was due lenz's law. But I'm still having a hard time to picture it. Please help me.

2020-05-25 20_07_03-transistors - Twice Vcc in transformer coupled Class A amplifier AC load lin.png
 

BradtheRad

Super Moderator
Staff member
Joined
Apr 1, 2011
Messages
13,473
Helped
2,667
Reputation
5,328
Reaction score
2,573
Trophy points
1,393
Location
Minneapolis, Minnesota, USA
Activity points
100,744
When you turn on the transistor fully, it provides a low-resistance path to a supply rail, causing that supply rail to influence the output.

When you turn off the transistor, it is high resistance, allowing the opposite supply rail to influence the output.
 

schmitt trigger

Advanced Member level 5
Joined
Apr 17, 2013
Messages
2,645
Helped
843
Reputation
1,690
Reaction score
819
Trophy points
1,393
Activity points
17,750
I had some spare time (Thanks COVID-19) and breadboarded a 2N3904 single ended class-A amplifier with an audio transformer load on the collector.

Indeed, the collector voltage exceeds B+, as shown in the attached scope waveforms.
The pink trace is the supply voltage (9v), the blue is the emitter voltage which bounds the lower swing, and the yellow is the collector voltage which goes above and below the battery voltage.

Transistor biasing is not optimized, and that is the reason the waveform will clip on the negative excursion and thus the full peak to peak excursion wasn't realized.
 

Attachments


paulmdrdo

Full Member level 3
Joined
Jan 17, 2014
Messages
175
Helped
1
Reputation
2
Reaction score
1
Trophy points
18
Activity points
1,365
When you turn on the transistor fully, it provides a low-resistance path to a supply rail, causing that supply rail to influence the output.

When you turn off the transistor, it is high resistance, allowing the opposite supply rail to influence the output.
HOw is the VCE able to vary from 0V to twice of the supply voltage?

- - - Updated - - -

I had some spare time (Thanks COVID-19) and breadboarded a 2N3904 single ended class-A amplifier with an audio transformer load on the collector.

Indeed, the collector voltage exceeds B+, as shown in the attached scope waveforms.
The pink trace is the supply voltage (9v), the blue is the emitter voltage which bounds the lower swing, and the yellow is the collector voltage which goes above and below the battery voltage.

Transistor biasing is not optimized, and that is the reason the waveform will clip on the negative excursion and thus the full peak to peak excursion wasn't realized.
Yes I know that would happen but my question is hOw is the VCE able to vary from 0V to twice of the supply voltage?
 

BradtheRad

Super Moderator
Staff member
Joined
Apr 1, 2011
Messages
13,473
Helped
2,667
Reputation
5,328
Reaction score
2,573
Trophy points
1,393
Location
Minneapolis, Minnesota, USA
Activity points
100,744
Does your single-ended amplifier have a capacitor somewhere in the output stage? If combined with an inductor (or transformer), then it's possible for LC resonant action to generate unexpected large voltage swings... particularly if no load is attached.
 

Easy peasy

Advanced Member level 5
Joined
Aug 15, 2015
Messages
2,484
Helped
912
Reputation
1,824
Reaction score
907
Trophy points
113
Location
Melbourne
Activity points
14,027
No - it is just the fact that once the xtor has turned on, i.e. output goes low, you have stored an amount of energy in the xfmr core, now if the xtor is turned off above a certain rate its o/p can go higher than the rail - thanks to the stored energy - this only works above a certain freq - if you do it too slowly you don't get this effect.

this is poorly explained in just about every text book.
 
Toggle Sidebar

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top