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 I_CQ = 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

[Mr Evil]

Design a class A common emitter amplifier using a BC109 with a voltage gain of 5. Modify the bias to produce class B operation...

Maybe they mean to first bias the transistor with a current source, then replace the current source with a complementary transistor, turning it into a push-pull amplifier?

[coldfiremc]

Class B stands for 180 degree conduction. So the bias must:

-Being on, only in the positive or negative hemicycle
-Have approximately 1 in voltage Amplitude gain (not mean, just when it's on).

For that, you have to turn your amplifier into a switchable current source, emitter output to minimize distortion.

[mackrc1]

Its seems the circuit is meant to stay the same and the it should only conduct for 180 degrees.

I had previously tried setting the bias voltage to 550mV were the transistor is nearly fully off but   with such a small input signal (100mV) the transistor is barely turning on and surley there would be a large amount of distortion as well?
Unless Im over thinking it and this is what I was meant to realise?

It seems to me that with such a small input signal it is not possible to have a true class B amplifier (where Iq = 0) at-least with the BC109.

[edavid]

How do you know what the input level is supposed to be?

And yes, you are probably supposed to observe the distortion level of the class B output.

[mackrc1]

The class A design wants a voltage gain of 5 with a 100mV input, then you modify the bias to class B, AB and C operation.
Sorry, I thought I had included it, I re worded it a few times before posting.