# Multi-Stage Amplifier Project

#### bigyanengi

Joined Feb 24, 2024
37
The lower end of Rb and R1 should go to the negative supply, not ground. The top of Ra should go to ground.
How did you calculate R1? Isn't it supposed to have 5mA through it?
Then do the same for Q8, to set the output current.
Hi Ian, Looks like I got what I want. Is this seems to be correct now?

#### bigyanengi

Joined Feb 24, 2024
37
The lower end of Rb and R1 should go to the negative supply, not ground. The top of Ra should go to ground.
How did you calculate R1? Isn't it supposed to have 5mA through it?
Then do the same for Q8, to set the output current.
Hello Ian,
Actually I did missed some more and now is good. Please see the screenshot. I calculated R1 based on these assumptions
The tail current is set by Rtail in series with Q5, and it's essential to ensure this current is less than 5mA. Assuming Q5 is a transistor acting as a constant current source for the differential pair:

1. Determine the Voltage Across Rtail:
• Assuming Q5 is an NPN transistor and the supply voltage (VCC) is +5V, the voltage across Rtail would roughly be VCC−VBE where VBE is typically about 0.7V for silicon BJTs in the active region. Thus, the voltage across Rtail would be approximately 5V−0.7V=4.3V
2. Calculate Rtail:
• For a maximum tail current of 5mA, use Ohm's law (V=IRV=IR) to find Rtail:
• Rtail=V/I=4.3V/5mA=860Ω
• This calculation provides the resistance needed to ensure a tail current of exactly 5mA. To guarantee it stays under 5mA,

#### Ian0

Joined Aug 7, 2020
10,044
Hello Ian,
Actually I did missed some more and now is good. Please see the screenshot. I calculated R1 based on these assumptions
The tail current is set by Rtail in series with Q5, and it's essential to ensure this current is less than 5mA. Assuming Q5 is a transistor acting as a constant current source for the differential pair:

1. Determine the Voltage Across Rtail:
• Assuming Q5 is an NPN transistor and the supply voltage (VCC) is +5V, the voltage across Rtail would roughly be VCC−VBE where VBE is typically about 0.7V for silicon BJTs in the active region. Thus, the voltage across Rtail would be approximately 5V−0.7V=4.3V
2. Calculate Rtail:
• For a maximum tail current of 5mA, use Ohm's law (V=IRV=IR) to find Rtail:
• Rtail=V/I=4.3V/5mA=860Ω
• This calculation provides the resistance needed to ensure a tail current of exactly 5mA. To guarantee it stays under 5mA,
View attachment 316093
No. What's the voltage at Vb1?
(and no, you haven't got a useful output - you have a peak-to-peak output current of only 700nA)

#### bigyanengi

Joined Feb 24, 2024
37
No. What's the voltage at Vb1?
(and no, you haven't got a useful output - you have a peak-to-peak output current of only 700nA)
The voltage is -3.001514v now

#### Ian0

Joined Aug 7, 2020
10,044
You have -3V the base, so you have -3.6V on the emitter, so you have 1.4V across R1.
R1 is 860Ω so current is 1.4V/860Ω, and that's not 5mA.

#### bigyanengi

Joined Feb 24, 2024
37
You have -3V the base, so you have -3.6V on the emitter, so you have 1.4V across R1.
R1 is 860Ω so current is 1.4V/860Ω, and that's not 5mA.
Hello Ian, when I calculated R1 was based on the 5mA current going assumption as I sent you the working before. So, decrease R1 to get close to 5mA current?

#### Ian0

Joined Aug 7, 2020
10,044
Hello Ian, when I calculated R1 was based on the 5mA current going assumption as I sent you the working before. So, decrease R1 to get close to 5mA current?
I get 1.6mA not 5mA.

#### bigyanengi

Joined Feb 24, 2024
37
I get 1.6mA not 5mA.
Hello Ian,
What I need to change to get close but not greater to 5mA? How about now? I changed the R1 to 300 ohm.

#### Attachments

• 104.5 KB Views: 2

#### Ian0

Joined Aug 7, 2020
10,044
That's better.
Now, you want 1mA in the next stage, so repeat the process for Q8.

#### bigyanengi

Joined Feb 24, 2024
37
That's better.
Now, you want 1mA in the next stage, so repeat the process for Q8.
Hello Ian,
Thank you for the support and staying with me here. Do I measure Ic of Q6 or Q7 to get that less than 1 mA? Plus the bias of Q6 is ok?

#### Ian0

Joined Aug 7, 2020
10,044
You have the same configuration as Q5, so the calculations are exactly the same, except you want 1mA not 5mA, so work out R3

#### bigyanengi

Joined Feb 24, 2024
37
You have the same configuration as Q5, so the calculations are exactly the same, except you want 1mA not 5mA, so work out R3
Hello Ian,
I have done the calculation and with 1.5k ohm as R3 my collector current will be less than 1mA. However, I do not see a curve for Ic on Q8. What might have happened there?

#### Ian0

Joined Aug 7, 2020
10,044
Now you need to sort out the output stage. Get rid of Rc Rd Re Rf Q6 and Q7, and add a PNP common emitter stage. (The cascode will come later - personally, I think it's pointless, but it's in the requirements)

#### bigyanengi

Joined Feb 24, 2024
37
Now you need to sort out the output stage. Get rid of Rc Rd Re Rf Q6 and Q7, and add a PNP common emitter stage. (The cascode will come later - personally, I think it's pointless, but it's in the requirements)
Hello Ian,
When I delete all existing Q6 and Q7 with their bias registers, the output from the first stage goes to the base of the pnp?

#### Ian0

Joined Aug 7, 2020
10,044
Perfect. You can see you have LOTS of gain.
Now, just because you have to, add in the cascode. (I assume you meant cascode, as you said cascade)

#### bigyanengi

Joined Feb 24, 2024
37
Perfect. You can see you have LOTS of gain.
Now, just because you have to, add in the cascode. (I assume you meant cascode, as you said cascade)
Hello Ian,
With all these curves i got from the simulation. Is everything is set the right order? And How do I add the third stage?

#### bigyanengi

Joined Feb 24, 2024
37
Hi,
Solve this problem first

Hint: where is the Emitter resistor for Q5
E
Hello Eric,
Sorry to bother you and you must be busy. IF you get a chance please look into my progress so far let me know If I am missing anything or of course from the assignment direction.

#### ericgibbs

Joined Jan 29, 2010
19,008
Hi big,
Those square wave plots are due to amplifier saturation of the 10mV Sine wave input.
The Current mirror type of load you are using for the Q1 and Q2 BJT's will give a VERY high gain.
Your circuit needs negative feedback to reduce the gain and make the amplifier stable.
E

#### bigyanengi

Joined Feb 24, 2024
37
Hi big,
Those square wave plots are due to amplifier saturation of the 10mV Sine wave input.
The Current mirror type of load you are using for the Q1 and Q2 BJT's will give a VERY high gain.
Your circuit needs negative feedback to reduce the gain and make the amplifier stable.
E
Hi Eric,
Thank you for your reply. I will apply negative feed back from the output all the way back to the V1 note or to the output of first stage?

#### Ian0

Joined Aug 7, 2020
10,044
Hello Ian,
With all these curves i got from the simulation. Is everything is set the right order? And How do I add the third stage?
Before you add the third stage, you need to add the cascode (it's pointless, all it will do is reduce the output voltage, but it's in the assignment). Then you needs to add the emitter followers.
Look up "Complementary emitter follower".
Then you need to check you have enough gain and bandwidth before proceeding to part 2 in which you add feedback (as @ericgibbs said in post #38)
What time zone are you in? Perhaps you could fill in the details. Some of us might have been outside enjoying the Spring sunshine!