Calculating transistor bias currents

In summary, the author is trying to find out whether their working method was correct as they have doubts with some of it. They first get the base voltage of two transistors, and then use the voltage division equation to get the emitter voltages of the two transistors. They then use the Ie = Ib + Ic relation to get the collector currents of the two transistors. Next, they are confused about how to get the current from the base to the collector of one transistor. They think they can get the current from the base to the collector by using the voltage from the power supply to the base of the transistor. However, neglecting the base current, the current going through the resistance (Rc) must also be
  • #1
andthenwecan
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Homework Statement


http://img846.imageshack.us/img846/955/29278644.jpg

The Attempt at a Solution



Basically I want to know whether my working method was correct as I have doubts with some of it.

I first did voltage division at the two base resistances, and I get 3V at the base. Since we know there's a base-emitter drop of 0.7V, the emitter voltage of Q1 is 2.3V which is also the base voltage of Q2, therefore the emitter voltage of Q2 is 1.6V.

This means we can calculate the emitter current of Q2 easily. After obtaining the emitter current of Q2, using the Ie = Ib + Ic relation and substituting for Ib = Ic/B, we can easily getting the collector current ic2.

http://img542.imageshack.us/img542/8632/80235300.jpg

Now here is where I'm a bit confused as to how to proceed and get ic1

I thought I can get the current from Vcc to Vb, let's call it ix, going back to the Vcc node, can I use that as a KCL point? So basically if I have ix leaving that node, and let's call the other current il which is going through Rc, those two must add up to give 0.

Once I have il, then il = ic1 + ic2 and therefore I can solve for ic1 and hence also find the voltage at Vc which is the DC voltage they asked for.

http://img6.imageshack.us/img6/664/12595537.jpg

Thanks.
 
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  • #2
For a given transistor, IB and IC, and IB and IE are related through the β; you should know these expressions.

Since you have calculated a value for the emitter current of Q2, you can work back to its base current. But that base current is also the emitter current of Q1... carry on.
 
  • #3
Ah...I see now. Yes, I know the expressions, I didn't read the question correctly: "neglect base currents where appropriate" - I completely neglected it.

Thanks for that.
 

1. What is a transistor bias current?

A transistor bias current is a small, steady current that is applied to the base of a transistor to keep it in the correct operating region. It is necessary for proper transistor function and to prevent distortion in the output signal.

2. Why is it important to calculate transistor bias currents?

Calculating transistor bias currents is important because it ensures that the transistor is operating at the correct point on its characteristic curve. This helps to maintain stability and prevent distortion in the output signal.

3. How do you calculate transistor bias currents?

To calculate transistor bias currents, you need to know the supply voltage, the transistor's base-emitter voltage, and the desired collector current. Using these values, you can use Ohm's Law to calculate the required base resistance and the bias current.

4. What are the different types of transistor biasing?

The three main types of transistor biasing are fixed bias, self-bias, and voltage-divider bias. Fixed bias involves connecting a fixed resistor between the base and the power supply. Self-bias uses a resistor and a capacitor to create a feedback loop. Voltage-divider bias involves using two resistors to create a voltage divider circuit.

5. How does temperature affect transistor bias currents?

Temperature can have a significant effect on transistor bias currents. As temperature increases, the base-emitter voltage also increases, resulting in a higher bias current. This can lead to thermal runaway and potentially damage the transistor. To compensate for this, temperature-compensated biasing techniques are often used.

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