Problem of adding active load to CECB folded cascode

Thread Starter

bstdms

Joined Jun 14, 2023
17
Hi,

I want to replace the output resister of a CECB cascode with a PNP active load. I did simulation on LTSpice.
The following is the cascode circuit and it worked as expected.

1714561145532.png

Then I replaced R1 with a PNP as active load. I biased the pnp directly by a voltage source and by the resistive network, respectively. But neither worked. I checked the DC operational point, it showed that VC2 is almost the same level as VCC(19.9753V) no matter which parameter I change. Thus the pnp bias was wrong.

1714562186735.png1714562230384.png

May I ask which part did I do wrong? Thank you.
 

Jony130

Joined Feb 17, 2009
5,539
The PNP current is not well defined in this case as well as the DC voltage at Vout. You need to make a real current from PNP. Thus, you need to add an emitter resistor. And properly biased the PNP base.
But the second problem remains, DC voltage at Vout is not well defined. So you need to add some negative feedback mechanism that will correctly set the output voltage at the desired level.
 

Thread Starter

bstdms

Joined Jun 14, 2023
17
The PNP current is not well defined in this case as well as the DC voltage at Vout. You need to make a real current from PNP. Thus, you need to add an emitter resistor. And properly biased the PNP base.
But the second problem remains, DC voltage at Vout is not well defined. So you need to add some negative feedback mechanism that will correctly set the output voltage at the desired level.
Thank you for the reply!
I added an emitter resistor and configured the output DC voltage using a resistive network(R7 and R8). The DC opreational analysis showed that all three resostors should be working in the active mode now.
1714606946938.png
1714606911393.png

My purpose is to reach a very high voltage gain by replacing Rc with the output impedance of the pnp(ro). However, the simulation result gave a only around 200 voltage gain, I think it's because R7//R8 dominates as a load resistor.
Could you provide any idea on how to properly bias the output DC voltage so that the pnp could work as an active load?

For the topology, I referred to this slide:
1714607701720.png
 

Jony130

Joined Feb 17, 2009
5,539
My purpose is to reach a very high voltage gain by replacing Rc with the output impedance of the pnp(ro).
I see, but what will be the load? What device will "receive" the amplified signal?
Also, why do you use BFP842? This is an RF transistor with Vce_max = 3.25V. So it cannot be used with Vcc = 20V.
Take a look at this example and the simulation file:
 

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Thread Starter

bstdms

Joined Jun 14, 2023
17
I see, but what will be the load? What device will "receive" the amplified signal?
Also, why do you use BFP842? This is an RF transistor with Vce_max = 3.25V. So it cannot be used with Vcc = 20V.
Take a look at this example and the simulation file:
Thank you very much for the solution!
The second stage is a commercial opamp in negative feedback setting, so the load resistor should be the input resistance of the opamp which is very large.
The reason I used HBT transistors is that they can work in very low temperatures which is required by my project. Also since the working temperature is low, I want to refrain from diodes.
I modified the circuit as follows, the output swing showed cutoff at 0.8V and -2.4V. Sorry for being silly, I don't know where does those two values come from and may I ask how to increase the output range?
1714678807324.png
 

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Jony130

Joined Feb 17, 2009
5,539
I modified the circuit as follows, the output swing showed cutoff at 0.8V and -2.4V. Sorry for being silly, I don't know where does those two values come from and may I ask how to increase the output range?
Add a big capacitor between U3 collector and GND.
 

Thread Starter

bstdms

Joined Jun 14, 2023
17
Why does my circuit oscillate?
I built the circuit on the breadboard.
1714908685383.png

Without any input, U1 collector showed a square-shaped oscillation with a frequency of 600Hz. This frequency can change when I press the components in the boards. I checked the DC voltage on each node and they all seemed normal.

1714908506167.jpeg

Is this due to the noise get amplified by the circuit? since this system is supposed to have an extremely large gain.
 

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Jony130

Joined Feb 17, 2009
5,539
I build this version on the breadboard using BC547B/BC557B.
cscode1.PNG

And no sign of oscillations. Except for 50 Hz noise.
The DC voltage at the collector:
RigolDS12.png

And the voltage across the R8 (load) for Vin = 4mVpp (1KHz)

RigolDS13.png

As you can see no sign of oscillations. And the voltage gain is around 1.5k [V/V].
Of course, I'm aware that the transistors I used are much much slower than yours.
Also, a cascade amplifier is prone to oscillation especially when you use HF transistors. But I would expect to see HF oscillations, not the LF ones.
So first try to remove R6 and C4 from your circuit. And try to increase C1. And see if this change the oscillation frequency.
You could also add a RC filter at the supply rail (10R and 680uF).
 
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