Calculate R3 in Transistor Circuit

[Harrison G]

Homework Statement

Hello, guys! I just wanted to make sure I do this right.

Homework Equations

Forget about the rest of the circuit and focus only on Q2 and Q4. How to calculate R3?

The Attempt at a Solution

The way I do it:
R3 is basicaly the emitter resistance which purpose is to regulate Hfe. So in most cases the emitter is held at 1V at current of 1mA. We find R3 from the formula R3=1V/1mA = 1000ohms. When Q4 is connected to the emitter of Q2, the emitter will fall to 0,6V and the current through R3 will reduce, leaving room for the base current of Q4 I3=0,6/1000= 0,0006A=0,6A. I3 reduced from 0,001A to 0,0006A. Now we can find what the base current of Q 4 will be from IB=IE-I3=1mA-0,6mA=0,4mA.

 

[tech99]

I think R2 should equal R3 so that there is equal and opposite drive into the push-pull pair Q3 and Q4.

 

[Harrison G]

I think R2 should equal R3 so that there is equal and opposite drive into the push-pull pair Q3 and Q4.

But the calculations for R3 are done in such a way like I did, isn't that right?

 

[NascentOxygen]

R3 is basicaly the emitter resistance which purpose is to regulate Hfe. So in most cases the emitter is held at 1V at current of 1mA. We find R3 from the formula R3=1V/1mA = 1000ohms. When Q4 is connected to the emitter of Q2, the emitter will fall to 0,6V and the current through R3 will reduce, leaving room for the base current of Q4 I3=0,6/1000= 0,0006A=0,6A. I3 reduced from 0,001A to 0,0006A. Now we can find what the base current of Q 4 will be from IB=IE-I3=1mA-0,6mA=0,4mA.

In order to follow your explanation, I think we need each mention of emitter to be identified as of Q2, Q3 or Q4.

 

[Jony130]

R3 is more a pull-down resistor. In this type of a circuit we do not care about Hfe much.
If you have time try watch this lecture