linear amplifier with voltage-divider bias

[mockingbird]

My lab homework is to use Multisim to design a linear amplifier with voltage-divider bias. We are using an example in the textbook as a template. Values for Vcc and the first four resistors were given from an example we did in the previous chapter. It looks like it should be easy, but my Vc isn't right. Here is a screenshot:

View attachment 63420

If I let it run long enough, Vc actually starts to approach 0. Whatever I did wrong is beyond my current level of comprehension. Will anyone please help me troubleshoot this?

 

[ronsimpson]

Problem-1
Your input signal is 10 volts peak, or 20 volts p-p.
Your supply for the amplifier is 10 volts.
The gain is about 2.
10 volts in 20 volts out peak with only a 10 volts supply.

Change the input signal to 1 volt pk and see what happens.
1V in pk, 2V p-p in
gain=2
output = 2V pk or 4V p-p.

Problem-2
The DC model shows almost 0 volts across the transistor, C to E.

You need to change the bias point.
Cut R2 in half. (2200 approximately). You need voltage across C to E.

 

[Roff]

Nodes 2 and 3 need to be connected together.

 

[mockingbird]

Thank you both for the responses. I have made (I think) the recommended alterations to my circuit, with unsuccessful results. How do I establish V[SUB]CE[/SUB]? In my notes, I have V[SUB]C[/SUB]=V[SUB]CC[/SUB]-I[SUB]C[/SUB]R[SUB]C[/SUB]. Are you saying the transistor is operating in saturation?

View attachment 63488

 

[Roff]

It still looks like you need to attach the voltage divider to the base. See attachment.

 

[JimB]

Whatever I did wrong is beyond my current level of comprehension. Will anyone please help me troubleshoot this?

At the risk of appearing a bit harsh, I am not surprised with that schematic.
How is any one supposed to read that!

OK, bear with me, you seem to be making mistakes which are common in beginners by having wires un-necessarly crossing all over the place, and components which do not line up, leading to dog-legs in the wires.
All this contributes to a lack of clarity and understanding.

Please look at my redraw of your simulator schematic, I hope that you find it easier to understand than your original.

JimB

 

[audioguru]

It would be nice if you turn off the dots on the schematic that are all over the place.

 

[mockingbird]

Not harsh at all JimB, for my level of experience. Your redraw is easier to understand. I find it easier to draw these by hand. If I was better with drawing in Multisim, I might have caught on to what Roff was saying earlier.

I have to be at work in 7 hours and I am up way past my bedtime studying circuits...

View attachment 63571

I have added yet another dot to the circuit, this one connecting the base to the voltage divider, as was kindly suggested earlier. I haven't checked the math, but the output looks like what I expected from the beginning.

 

[JimB]

Still a few odd unnecessary kinks in the wires, but much better and more understandable. Keep it up.

JimB

 

[Winterstone]

Thank you both for the responses. I have made (I think) the recommended alterations to my circuit, with unsuccessful results. How do I establish V[SUB]CE[/SUB]? In my notes, I have V[SUB]C[/SUB]=V[SUB]CC[/SUB]-I[SUB]C[/SUB]R[SUB]C[/SUB]. Are you saying the transistor is operating in saturation?
View attachment 63488

I am afraid you forgot the dc drop Ic*Re across the emitter resistor, didn't you?

 

[audioguru]

Why do people use Multisim? Only because their teachers do?
Multisim makes schematics with the parts spread out too much, it dosn't know about many modern parts and it produces a schematic with wires having many un-necessary turns and has dots all over the place.