Electronics > Beginners
Class B biasing
mackrc1:
Hi
I am biasing a class A common emitter amplifier to operate in class B mode but i am not sure about my calculations and thought process.
A class B amplifier is biased so Vce = vcc and Icq = 0. I assume that in reality Vce = close to vcc and Icq = close to zero.
I am using a BC109
Vcc = 9V
Vce = 8.95V
Icq = 1uA
Rc = 0.05/1uA = 50k
Re is typically 10% of vcc for class A but not sure for class B, I assume bias stability is still necessary?
Ve = 0.9V
Re = 0.9/1uA = 900k
I have no gain now but i can use a bypass capacitor on the emitter to meet that.
Vbe typically is 0.7 but since im operating at cutoff I assume the lowest figure 0.55V is that correct?
Vb = 0.9 + 0.55 = 1.45V
This method works from my simulations but i am unsure if it is the correct way. :-// This is an assignment, it is not a push pull amplifier.
Cheers
Benta:
I'm not quite sure what you mean. The term "class B" usually only makes sense for complementary output stages.
If you're trying to make a 1-transistor CE stage, then reducing the steady-state CE current will just reduce its output drive capability.
mackrc1:
From what i can understand I need to modify a class A CE amplifier so that it performs like one of the transistors in a push pull class B amplifier.
edavid:
If the assignment specifies ICQ = 0, there should be no base bias.
We can't tell from what you've said if there should be an emitter resistor. Post the rest of the assignment.
mackrc1:
Design a class A common emitter amplifier using a BC109 with a voltage gain of 5. Modify the bias to produce class B operation. Goes on to to ask for class AB and C as well but thats another story. Thats all there is, no Icq or Vce
I think it means something like this, which is my best effort so far, I'm just unsure of my design method
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