Author Topic: Class B Amp  (Read 1543 times)

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Offline Steve1959Topic starter

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Class B Amp
« on: April 03, 2019, 02:36:28 am »
I was taught that a class b amp (transistors) is usually in a push-pull configuration. Therefore requiring two.
I have a circuit for a transistor demodulator for AM and it says it is wired for class B. Can a single transistor be wired class B or is it a typo? Find attached a .jpg to see the circuit.

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« Last Edit: April 03, 2019, 02:38:22 am by Steve1959 »
 

Offline hamster_nz

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Re: Class B Amp
« Reply #1 on: April 03, 2019, 03:23:57 am »
Looks to be typo. That is a common emitter amplifier.

Unless something funny is going on and the transistor is biased to cause non-linear operation.
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Offline Calvin

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Re: Class B Amp
« Reply #2 on: April 03, 2019, 06:02:05 am »
Hi,

Quote
Can a single transistor be wired class B or is it a typo?
A single transistor can be biased in class B (or class A and C) however You wish.
The classification only describes over how much part of 360° of a phase the transistor conducts.
Class B is defined as 180°, class A as 180° to 360°, class C 0° to 180°.
In this case however it seems rather a typo, as it wouldn´t make much sense to bias this amplifier below class A

regards
Calvin
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Offline Gyro

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Re: Class B Amp
« Reply #3 on: April 03, 2019, 08:54:13 am »
It's hard to tell without component values but the placement of C2 is unusual. In combination with R3 this will form a time constant which will limit the rate of rise of the output when Q1's Base is being driven low by the input waveform. When the Base is driven high, it will form a lower resistance path for charging C2 again.

It would be wrong to describe it as Class-B amplifier though as there's always going to be some C-E current flowing through Q1, even if it varies a lot more than usual for a common emitter stage (again, depending on component values).
« Last Edit: April 03, 2019, 08:56:43 am by Gyro »
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Offline mikerj

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Re: Class B Amp
« Reply #4 on: April 03, 2019, 12:13:36 pm »
I suspect this really is class B (or very close to) as the biasing resistors will be set to barely turn on the transistor.  It's effectively a half wave rectifier with some gain and close to zero forward voltage due to the biasing.
 

Offline Circlotron

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Re: Class B Amp
« Reply #5 on: April 03, 2019, 12:55:04 pm »
^^ And therefore an AM detector. A normal diode detector also has 180 deg conduction angle.
 

Offline Steve1959Topic starter

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Re: Class B Amp
« Reply #6 on: April 03, 2019, 10:06:08 pm »
mikerj,  You were correct. I did some more digging. It is biased very low so it will act like a diode. Only positive pulses will be produced. C2 filters out the carrier and the modulation is coupled out at C3. Thanks

Thanks everyone for the assist.
« Last Edit: April 03, 2019, 10:08:54 pm by Steve1959 »
 

Offline AlanS

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Re: Class B Amp
« Reply #7 on: April 04, 2019, 12:28:24 am »
Mmm.

With some values we could run it through a simulator! Hell we could make up some values and run it though a simulator. :scared:
 

Offline hamster_nz

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Re: Class B Amp
« Reply #8 on: April 04, 2019, 12:52:21 am »
Mmm.

With some values we could run it through a simulator! Hell we could make up some values and run it though a simulator. :scared:

Pulling numbers out of thin air...

Vcc=5V, 43k Ohm, R1=150k & R2=22k, R3=1k seems to be about right for something like Class B operation....
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