# help with transistor design

Discussion in 'Homework Help' started by vg30dett, May 15, 2011.

1. ### vg30dett Thread Starter New Member

May 15, 2011
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Greetings to all. I am a new member of this forum. I am trying to design a single state common emitter transistor with the following initial conditions:

Vcc=20V
Ic(max) = 120 mA
Vc=8V
B=120

What should I do to start? I am looking at a bunch of formulas, but am unsure what the first step in this case should be.

Should I first plot the load line and determine the quiescent point?

Any help would be greatly appreciated.

GTXR

2. ### blah2222 Well-Known Member

May 3, 2010
554
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Well, for starters you know that Ic(max) is 120 mA and you are constrained to 8 V at the collector, so you can figure out the resistance of the resistor between supply and collector. I'm assuming you are biasing the transistor in a four-resistor network.

3. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
Setting Ic to 120mA seems somewhat extreme. Simple common emitter voltage amplifiers are general biased with Ic of a few mA. Perhaps Ic(max) refers to the rated value. One would keep well below this rated value.

The OP hasn't been given much in the way of typical design requirements for the amplifier. No mention of gain, output swing, minimum input resistance limitation and so forth....???

4. ### vg30dett Thread Starter New Member

May 15, 2011
5
0
Thank you all for the prompt replies:

The gain is 120. Ic max is a manufacturer rating. Hope this helps.

5. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
When you say the "gain" is 120 do you mean the transistor β value is 120?

There's a difference between amplifier voltage gain Av and the transistor current gain β.

Have you noted the "sticky" regarding the expectation that posters in the homework forum are expected to show their working. You've provided very little thus far.

6. ### vg30dett Thread Starter New Member

May 15, 2011
5
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(1) Yes, I am referring to beta.
(2) No where in my post have I asked anyone to do this for me. I do not know where to start. Looking for advice.

7. ### vg30dett Thread Starter New Member

May 15, 2011
5
0
Nevertheless, I will try to come up with something and post it.

Thank You.

8. ### Audioguru New Member

Dec 20, 2007
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896
Look at tutorials about simple transistor circuits on the internet again??

Hopeless students should be failed then maybe when they repeat the course they will learn something.

Hopeless teachers should also be failed. Then fired.

Dec 26, 2010
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A single state common emitter transistor? Do you mean a single stage amplifier?

Your first step should be to decide on some specifications for your amplifier - otherwise the problem is too open-ended.

What output level is required (how much voltage swing , and into what load)? Does this set the minimum power requirements?

Will the circuit have to cope with a range of different transistor gains or a wide temperature range? How could this be accommodated?

Do you require a specific output impedance? What about the input impedance?

What is the input level, or how much gain is required? What is the frequency range?

There may be other things to consider, but the point is to get your requirements well enough defined to get on with the job. Be careful though not to be too ambitious with the values, or for instance you may get a design which requires an impossibly high transistor gain.

10. ### vg30dett Thread Starter New Member

May 15, 2011
5
0

thank you for trying to help. These are the specs the professor gave. He just told us to draw a load line, and determine some starting values, then deduce everything from there.

11. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
Other than a few minor constraints you seem to have a completely free hand in this. That being the case why not set your own objectives.

1. Specify a gain range - say |Av|=50 to 75
2. Set a modest quiescent collector current - say between 2 & 5 mA.
3. Select the collector resistance based on Vcc=20 and Vc=8. Choose a preferred resistor value, rather than the exact value - a practical consideration.
4. Develop a stable biasing arrangement - you might use voltage divider biasing, since this is normally good practice to achieve bias stability. Again select preferred resistor values to make the exercise realistic.
5. Assume the amplifier has to drive a load - say 10kΩ
6. Design for the other circuit values.

If you are required to use a load line method you will need a graph of the characteristic curves for the transistor in question (???) - Ic vs Vce with various fixed Ib. Remember to include the load resistance value in the effective collector load.

Have you ever used the load line method? This technique, whilst perhaps instructive, is rarely used these days - one can develop a perfectly acceptable design without its use.

Feb 5, 2010
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