Multi-Stage Amplifier Project

Ian0

Joined Aug 7, 2020
10,039
@Ian0
DO I need to add another pnp transistor to do this modification? If I add lets say pnp Q9, and bias the base and connect the collector to 5v then what should I do with the emitter? I cant connect the emitter with the output coming from Q8 collector.
Don't forget that every example you see of cascode in textbooks uses NPN transistors (or N-channel JFETs), and your common emitter stage is PNP, so your common base stage also must be PNP. (It's upside down, compared to all the text book circuits)

Look at Figure 3.
 
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Thread Starter

bigyanengi

Joined Feb 24, 2024
37
Don't forget that every example you see of cascode in textbooks uses NPN transistors (or N-channel JFETs), and your common emitter stage is PNP, so your common base stage also must be PNP. (It's upside down, compared to all the text book circuits)
@Ian0
SO my Vout for this second stage remain unchanged? The emitter flower q8 maintain the current load so I think that needs to be there as well. I have been doing a lot of reading to make a sense of what you said "If you start with the post #41 circuit which has a common emitter output.
The cascode is a common emitter driving a common base - so the output of the common emitter circuit, goes to the INPUT of the common base circuit. The final output is from the output of the common base circuit (which connects to a constant-current load)" and yet I am not making any progress. The Out put from Q2-Q4 common junction should be relocated to which junction in this cascode stage? 1708888216022.png
 

Ian0

Joined Aug 7, 2020
10,039
Why have you reversed collector and emitter on Q6 and Q7? PNP transistors only work properly with their emitters more positive than their collectors.
It's the COMMON EMITTER STAGE that drives the COMMON BASE STAGE
You have it the other way round.
 

Thread Starter

bigyanengi

Joined Feb 24, 2024
37
@Ian0
SO my Vout for this second stage remain unchanged? The emitter flower q8 maintain the current load so I think that needs to be there as well. I have been doing a lot of reading to make a sense of what you said "If you start with the post #41 circuit which has a common emitter output.
The cascode is a common emitter driving a common base - so the output of the common emitter circuit, goes to the INPUT of the common base circuit. The final output is from the output of the common base circuit (which connects to a constant-current load)" and yet I am not making any progress. The Out put from Q2-Q4 common junction should be relocated to which junction in this cascode stage? View attachment 316177
@Ian0
I might have misunderstood your guidance earlier. I have made the changes.
1708889656100.png
 

Thread Starter

bigyanengi

Joined Feb 24, 2024
37
Why have you reversed collector and emitter on Q6 and Q7? PNP transistors only work properly with their emitters more positive than their collectors.
It's the COMMON EMITTER STAGE that drives the COMMON BASE STAGE
You have it the other way round.
@Ian0
The tail current source for Q5 and Q8, is there better and reliable ways to boas those transistors? I think I may have the hard time setting up the current I want in the actual circuit.
 

Ian0

Joined Aug 7, 2020
10,039
@Ian0
The tail current source for Q5 and Q8, is there better and reliable ways to boas those transistors? I think I may have the hard time setting up the current I want in the actual circuit.
Yes, there are, but it seems that you aren't allowed to use them.
Use two diodes in series to set a voltage of 1.2V (2.Vbe) on the base. That means the voltage across the resistor is Vdiode (about 0.6V). It doesn't have to be accurate to work.
 
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