Differential Amplifier design

Thread Starter

Rumination

Joined Mar 25, 2016
74


The figure shows a differential amplifier with a differential input vid = (vin_d_pos - vin_d_neg) between the bases of Q1 and Q2, and an output (out) at the emitter of Q4.

I have to Determine R1, R2, R4, R5 and R6 so that the following requirements are met:

R 1 = R 2
Av = vout / vid = 100 [V / V]
Vout = 0V when Vin_d_pos = Vin_d_neg = 0V.
The output Vout must be able to deliver undistorted peak signal voltage in the range
from vout-min to vout-max with RL armed.
Quiescent current IC = 10 * Q3-Q4 IB

----------
I have been given the values:
Vcc-pos = 21 V
Vcc-neg = 21 V
R3 = 2000 Ohm
RL = 49 Ohm
I1 = 3,56 mA
Vout-max = 8,8 V
Vout-min = -8,8 V
----------
How can I start? For I have difficulty getting started.
 

Thread Starter

Rumination

Joined Mar 25, 2016
74
My English is not so good. I have found this out:

Vcm = 0 , Vin_D = 0
VB1 = VB2 = 0 V.

Ic1 = IE1 = I/2 = 3,65 / 2 = 1,825 mA

VB3 = 8,8 V - R3 * Ic1 = 8,8 V - 2000 * 1,825 mA = 5,15 V.

Ic3 = IE3 = (24 V - VB3 - 0,7 V) / R4

But I don't know the value of R4?
 

Jony130

Joined Feb 17, 2009
5,005
Hey but, your Vcc-pos is 21 V so VB3 voltage is equal to (if we ignore IB3):
VB3 = 21V - Ic1*R3 = 21V - 1,825 mA*2kΩ1 = 21V - 3.65V = 17.35V
Also notice that R4 and R5 "set" the Q3 voltage gain.
Quiescent current IC = 10 * Q3-Q4 IB
I don't understand this ?
 

Thread Starter

Rumination

Joined Mar 25, 2016
74
Yes, you are right. It's 21 V and not 8,8 V. I got confused. But what does this mean:

"R4 and R5 "set" the Q3 voltage gain"?

And just ignore, Quiescent current IC = 10 * Q3-Q4 IB.
 

crutschow

Joined Mar 14, 2008
23,494
But I don't know the value of either R4 or R5?
But you know their ratio.
This is not a single answer to their correct values, only their ratio. You select the value to give the desired Q3 transistor bias current.
For a real design this current would affect the frequency response which may be of interest.
 

Thread Starter

Rumination

Joined Mar 25, 2016
74
But you know their ratio.
This is not a single answer to their correct values, only their ratio. You select the value to give the desired Q3 transistor bias current.
For a real design this current would affect the frequency response which may be of interest.
Do you mean:

Ad = gm * Rd ?
 

Jony130

Joined Feb 17, 2009
5,005
You must start at the output. You have been given RL and Vout_max so from there you can find Ic4 and Ic3 quiescent current.
And choose R4,R5R1,R1 to meet the voltage gain requirement at given quiescent current .
 

Thread Starter

Rumination

Joined Mar 25, 2016
74
Can I do it like this:

VB3=17.35V
We know: VE3=(17.35+0.7)V=21-IC3*R4
We also know: VB4=+0.7V=-21+IC3*R5.
So by eliminating IC3 it gives a second equation for R4 and R5:

18,05 = 21 - R4
R4 = 2,95

0,7 + 21 = R5
R5 = 21,7

Does it mean, that we have found the values of R4 and R5? And now we can find the value of Ic3?
 

Thread Starter

Rumination

Joined Mar 25, 2016
74
You must start at the output. You have been given RL and Vout_max so from there you can find Ic4 and Ic3 quiescent current.
And choose R4,R5R1,R1 to meet the voltage gain requirement at given quiescent current .
To find Ic3:

Ic3 = IE3 = (21 V - VBE - 0,7 V) / (R4) = (21 V - 17,35 V - 0,7 V) / (2,95) = 2,95 / 2,95 = 1 mA.

To find VB4:

VB4 = -21 V + R5 * Ic3 = -21 V + 21,7 V * 1 mA = 0,7 V.
Vout = VB4 - 0,7 V = 0 V

To find Ic4:
Ic4 = (0 - (-21 V)) / R6

But I don't know the value of R6.
 

Jony130

Joined Feb 17, 2009
5,005
It looks like you do not know (or understand) how to find maximum negative voltage swing at the load for a simply emitter follower circuit (Q4).
 

Jony130

Joined Feb 17, 2009
5,005
To help you with this problem look again at my post #14 and Ask yourself -- >What voltage at the T1 base will cut-off T1 transistor?
 
Top