# Design a cascode BJT amplifier to achieve a specified voltage Gain

I have been asked to design a cascoded BJT amplifier with the given specifications

• Voltage Gain = $$\76 \ to\ 84\ dB\$$
• Current Gain = $$\66.5 \ to \ 73.5\ dB\$$
• Power Dissipation = $$\ 3.5 \pm 5\% \ mW \$$

However , I also have to adhere to the following rules :

• Power supply rails available are$$\ V_{dd} = 3.3V\$$ and ground only.
• Only one current source can be used in the design $$\ I_{ref} = 50\mu A\$$
• Every bias voltage needed has to be generated by designing proper circuits.
• Only one resistor of value around $$\ 40k \Omega \$$ can be used.
• Output swing has to be atleast 80% of $$\V_{dd} \$$
• Use the model BC108B for NPN and BC178B for PNP transistor.

To just start off , I designed a rough schematic as follows , not necessarily following the rule to keep only one voltage source :

By plotting the voltage transfer characteristics of BC108B transistor , I concluded that to remain in the active region , the $$\ V_{be}\$$ has to be between $$\700mV \ and \ 900mV\$$ , so i went with $$\800 mV\$$ , hoping both the NPN BJT's would be biased in the active region and give me a high voltage gain.

However , when I do the transient analysis , the gain comes out to be less than 1 , and also the bias current as $$\ \approx 55 \mu A\$$

I believe the reason for this small gain might be that the NPN transistors are actually not biased in the active region , but i am not sure.

How do I get the desired voltage gain as a starting point while at the same time satisfying the requirements mentioned before ?

• Experiment a little with it in your simulator to see if you can improve it. I mean; you have got this far with the design so a few little tweaks shouldn't be much of a challenge. – Andy aka Apr 3 at 9:41
• You guys need to exchange notes ---> electronics.stackexchange.com/questions/558097/… – Andy aka Apr 3 at 9:50