Class B biasing - Page 1
Forum section: electronics / beginners

author: mackrc1
author_rank: Contributor
timestamp: February 22, 2020, 05:03:44 pm
content: HiI 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 BC109Vcc = 9VVce = 8.95VIcq = 1uARc = 0.05/1uA = 50kRe is typically 10% of vcc for class A but not sure for class B, I assume bias stability is still necessary?Ve = 0.9VRe = 0.9/1uA = 900kI 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 = 1.45VThis 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

author: Benta
author_rank: Super Contributor
timestamp: February 22, 2020, 05:21:16 pm
content: 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.

author: mackrc1
author_rank: Contributor
timestamp: February 22, 2020, 05:43:23 pm
content: 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.

author: edavid
author_rank: Super Contributor
timestamp: February 22, 2020, 06:18:36 pm
content: 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.

author: mackrc1
author_rank: Contributor
timestamp: February 22, 2020, 07:00:40 pm
content: 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 VceI think it means something like this, which is my best effort so far, I'm just unsure of my design method

author: Mr Evil
author_rank: Regular Contributor
timestamp: February 22, 2020, 08:35:11 pm
content: Quote from: mackrc1 on February 22, 2020, 07:00:40 pmMaybe 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?

author: coldfiremc
author_rank: Regular Contributor
timestamp: February 22, 2020, 08:58:26 pm
content: 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.

author: mackrc1
author_rank: Contributor
timestamp: February 22, 2020, 11:03:38 pm
content: 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.

author: edavid
author_rank: Super Contributor
timestamp: February 23, 2020, 02:04:47 am
content: 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.

author: mackrc1
author_rank: Contributor
timestamp: February 23, 2020, 08:16:30 am
content: 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.