FAQ: HMC396, HMC405, HMC311, HMC395 and HMC397 22 ohm resistor

Q: HMC396, HMC405, HMC311, HMC395 and HMC397 are die level Darlington amplifiers that recommend the use of a 22 ohm resistor in series with the drain bias inductor. What is the impact of varying the size of this resistor?  

  • +1
    •  Analog Employees 
    on Mar 1, 2021 3:20 AM 1 month ago

    A: Darlington amplifiers are ideally supposed to be biased by a current source.  So ideally, there should be a current source pumping a constant current into L1 (instead of Vs driving Rbias). In practice, we don’t do this. Instead, we can approximate a current source using a supply voltage (Vs) and a series resistor(Rbias). The bigger the value of Rbias that you use, the more ideal your current source is (an ideal current source has an output resistance of infinity). However, big resistors cause big voltage drops, giving less  bias voltage to the amplifier. This will tend to result in lower IP3 and P1dB. But if we make Rbias very small (or 0), we are now biasing the Darlington Amplifier with a voltage source. If you do this the circuit’s performance will vary a lot vs supply and vs. temperature. So we try to strike a balance and use a resistor which doesn’t cause  too much voltage drop but which is still big enough so that (Vs & Rbias) looks like a current source. That value was determined to be 22 ohms for all of amplifiers mentioned above.