# Homework Help: Simple question on differential amplifiers

1. Jan 15, 2014

### nil1996

1. The problem statement, all variables and given/known data
My text book says
"""When a dual power supply is used,the biasing resistors are so selected that the desired forward bias to the two transistors is provided,at the same time the voltages at the bases of the two transistors are brought down to ground potential.

Hence when the signal source is connected between input1 and the ground,there is no change in the operating point of the transistors,because base is already at the ground."""

in the first paragraph,
if the base voltage is at ground how will the transistors work then?

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Last edited: Jan 15, 2014
2. Jan 15, 2014

### Staff: Mentor

The bases are set at ground potential by the bias network when there's no signal present. When a signal is present the base will vary from 0V accordingly.

3. Jan 15, 2014

### CWatters

4. Jan 15, 2014

### nil1996

I not still getting it It is said that the resistors R1 and R2 are so selected that the transistors are in forward bias and the bases are at ground potential. Forward biasing means getting some voltage at the base so that a current would flow through the base.But grounding means making potential at the base '0'.So then how can we get the forward bias.

(sorry if i am being too dumb,but i want to understand it in deep:( )

5. Jan 16, 2014

### rude man

This is a strange circuit. The resistors R1-R4 do nothing! When you operate such an amplifier you are applying the voltages you want to the bases. The resistors are then meaningless. All they do is radically reduce the input impedance, something you don't want to do!

Base biasing currents are provided by the two applied input voltage sources. These can include zero and zero of course.

Also, the two powers supplies need to be refrenced to ground (connect - of Vcc to + of Vee).

Last edited: Jan 16, 2014
6. Jan 16, 2014

### nil1996

I am not understanding how you came to that conclusion .Will you please explain how R1 and R4 do nothing and also how they reduce the input impedance.

7. Jan 16, 2014

### CWatters

The absolute voltage on the base does not matter , what matters is that the base is higher than the emitter. In this case the base is at 0V and the emitter at about -0.7V.

8. Jan 16, 2014

### nil1996

O,got it.
thanks a lot.

but still not understood why rude man said that r1 and r4 do nothing

9. Jan 16, 2014

### rude man

If you ground an input, what is the voltage at that input? It doesn't matter what R1 and R2 are! But the source has to supply extra current to those resistors. Same for R3 and R4.

If you apply x volts at an input, what is the voltage at that input? Same answer!

Without the resistors the input impedance is high (small base currents). With the resistors the (ac) impedance is less than R1||R2 for the left input and R3||R4 for the right.