This version is outdated by a newer approved version.DiffThis version (03 Jan 2021 21:58) was approved by Robin Getz.The Previously approved version (14 Aug 2017 08:30) is available.Diff

This is an old revision of the document!

CN0273: High Speed FET Input Instrumentation Amplifier with Low Input Bias Current and High AC Common-Mode Rejection


The circuit is a high speed FET input, gain-of-5 instrumentation amplifier (in-amp) with a wide bandwidth (35 MHz) and excellent ac common-mode rejection, CMR, (55 dB at 10 MHz). The circuit is ideal for applications where a high input impedance, fast in-amp is required, including RF, video, optical signal sensing, and high speed instrumentation. The high CMR and bandwidth also makes it ideal as a wideband differential line receiver. Ideal as a wideband differential line receiver.


  • EVAL-CN0273-EB1Z circuit evaluation board
  • +/-5 V dual power supply
  • Signal generator with a balanced differential drive source
  • 3 SMA to BNC Connector


Power Supply

 +/-5V supply should be supplied to the connector for proper operation of the evaluation board  

Input Signal

 Signal Generator w/ Balanced Differential Drive Source can be used as an input source.

Peak to Peak Input Voltage Restriction

  • Maximum allowable differential input is ±3.5 V/G1, where G1 is the gain of the first stage.
There is a tradeoff between the maximum allowable differential input and the closed-loop gain of the first stage.

Common Mode Input Voltage Restriction

  • Allowable input common-mode range for the composite circuit is from −3.5 V to +2.2 V

Bandwidth Input Restriction

  • Allowable input frequency bandwidth is 35MHz @ -3dB, typical for G = 5

Output Measurement

 Oscilloscope can be used to measure the output of the evaluation board with a gain of 14db below 10Mhz input as indicited in the graph below.

The output swing of the circuit is limited to ±3.5 V when operating on ±5 V supplies


FET Amplifier Input Gain Stage

  • The circuit has a differential inputs gain of 1 + 2RF/RG
    • Where,
      • RF = Resistor R5 and R6
      • RG = Resistor R7
  • For this circuit, a feedback resistor of 200 Ω was chosen. The feedback capacitor, CF, was 2 pF for the best bandwidth flatness.
  • When the inputs are common-mode, there is no current flowing through the RG gain resistor. Thus, the circuit acts as a buffer for the common-mode inputs.


End of Document

resources/eval/user-guides/circuits-from-the-lab/cn0273.1609707339.txt.gz · Last modified: 03 Jan 2021 21:55 by Robin Getz