Run into Wide-Band Buffer/Amplifiers

June/1 2016

these several days i struggle to get a circuits use as sweeper post mixer amplifier. 

diode DBM required well terminated the IF port with  wideband 50 ohm terminator, isolate the IF filter (SWR change rapidly in pass band). and in the same time, sweeper output frequency from 100 khz to 45 Mhz, the amplifier should keep a stable gain from 100 kHz to 50 Mhz . 

this sweep generator use 60 Mhz beating 60 to 120 Mhz oscillator to get a  100 Khz to 45 Mhz sweep signal. the gain required keep flat up to 45 Mhz at least.  the input impedance better should be 50 ohm start from 100 khz extended to 200 Mhz or more.

Oscilloscope is one of such a devices required similar thing. following is the input buffer of  my old  Hitachi V1050F oscilloscope.

JFET buffer following with the CFA ( current feedback amplifier ), use the OP control the DC gain seems a 'standard' way for analog oscilloscope. 

Simulation such a amplifier, with some compensate to BJT's input  capacitance, theoretical bandwidth could reach 200 Mhz. 

The most advantage of the jfet buffer CFA is high input impedance. Ltspice make it very easy to see how the circuit works.  CFA, the Q2 BE voltage keep stable around 0.7 V,  input signal almost could not change it's voltage, so the miller capacitance keep small, bandwidth extended.

All of the result here is just simulation, make actual circuits achieve the same result as the Ltspice does, is  big challenge.