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I'm going to design amplifier, requirement is that impedance for in should be more than 20kohm, impedance for out should be less than 50ohm, and voltage gain should be more than 200. So I'll make cc-ce-cc amplifier, but how can I choose transistor for each cc, ce, cc? And is it better to chooose transistor first and after that choose Rc, Re...and then R1, R2...etc? Or is there any other better way(order) to design it? Any guide will be appreciated. Thanks

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    \$\begingroup\$ Specify the frequency range over which the gain is maintained. Also specify the peak to peak voltage input. When you write \$50\:\Omega\$ output impedance, do you mean to drive a \$50\:\Omega\$ input at the other end? (If so, your output voltage swing must be twice what you think, if the device your amplifier drives has a \$50\:\Omega\$ input and it must "see" a gain of 200 or more.) Does your output need to present \$50\:\Omega\$, both sinking and sourcing? Or is that only sourcing? Or only sinking? \$\endgroup\$ – jonk Nov 18 '18 at 2:41
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    \$\begingroup\$ Some last bits to consider and I'll leave this alone unless you clarify things. You've provided no compliance output voltage range. That's needed. I'd also consider a class-AB output stage, able to sink or source equally well and use emitter resistors to set up the output impedance as desired. This would have slightly less than Av=1 to it. If input is single-ended I'd consider a bootstrapped 1st stage to keep input impedance much higher than otherwise. It's gain depends on your voltage rails. In between, a VA stage. Then the output. \$\endgroup\$ – jonk Nov 18 '18 at 4:02
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Voltage gain for a single bipolar device, with no emitter resistor, is at most VDD/0.026. You want 200, thus you need at least 5 volts. Go for 12 volts.

For a Rin of 20,000 ohms, assuming Beta of 100, you need reac of 200, which means Ic of about 100 microAmperes.

To swing the collector voltage by 10 volts at 100uA, you need 100K ohm Rcollector.

To interface that to 50 ohms, you need a lot of emitter followers (CC topology).

Given Rcollector is 100Kohm, and assuming 20pf (two of Cob) on that node, your timeconstant is 2 microSeconds, 500,000 radians per second, or about 90,000Hertz F3dB.

Any small-signal bipolar will do fine: 2N3904, 2N918. Even Nchannel JFETs, if the transconductance is high enough.

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