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 : enter image description here

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 ?

  • \$\begingroup\$ 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. \$\endgroup\$ – Andy aka Apr 3 at 9:41
  • 2
    \$\begingroup\$ You guys need to exchange notes ---> electronics.stackexchange.com/questions/558097/… \$\endgroup\$ – Andy aka Apr 3 at 9:50

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