cascode Amplifiers

Discussion in 'Feedback and Suggestions' started by microgary, Feb 22, 2006.

  1. microgary

    Thread Starter New Member

    Feb 22, 2006
    I went to a job interview and encountered a diagram for a cascoded amplifier circuit. All of my old texts did not mention that circuit. An internet search uncovered a couple of references. My old Reference Data for Radio Engineers refered to a triode tube cascoded amplifier. I can see how the circuit would very important at reducing distortion to the amplified signal. A search of your database reveled no "hits" Any posibility of including this in your literature?
  2. Dcrunkilton

    E-book Co-ordinator

    Jul 31, 2004
    I will put it on my list. (I actually do have a list) However, as many readers have pointed out, there are already missing sections, topics, chapters in the various volumes for which header or titles are in place, in particular, the Semiconductors Vol. 3. In other words, don't hold yor breath over the next couple of years. It does sound like a good ieda, though.

    The modern version of the cascode amplifier is a pair of stacked (cascode) JFETS. Look for a Siliconix JFET manual. I don't know if that material in that old manual is online anywhere.

    The other modern version of Casocde is a dual gate MOSFET transistor, which can be wired as cascode. You can look at the dual gate MOSFET as being equivalent to two stacked (cascoded) MOSFETs.

    Cascode is used to achieve a higher frequency response than is possible than with a common emitter (common source or common cathode) amplifier. The problem with a common emitter (source) amplifier is the capacitance from collector to base (drain to gate) , also known as Miller capacitance, limits the high frequency response. The way you get around this is to go to a common grid for vacuum tubes, a common base for bjt, common gate for JFETS or MOSFETS. The grounded grid, base, or gate shields the input from the output: the cathode from the plate, the emitter from the collector, the source from the drain. The grounded grid, base, or gate gives much better high frequency response at the expense of having a very low input impedance. If it were not for the problem of low input impedance, this would be the solution. Try an older ARRL Manual, or other radio circuits book for more info on grounded grid, base, or gate amplifiers.

    The idea of cascode is to modify the grounded grid, base, or gate configurations by giving it a higher input impedance. This is done by feeding the emitter (source) input of the grounded base (gate) stage with the output of a common emitter (source) stage. The top transistor in a Cascode is a grounded base (gate) stage because the top base (gate) is at AC ground. And the input to the top stage comes in on the emitter (source) from the stage below. The bottom stage input is on the base (gate), is high impedance, is amplifed at the collector (drain) which is passed to the top stage emitter (source). More infomation on cascode may be found in Radio Frequency books.

    The clue that a dual gate MOSFET might be cascode is at the top gate input. Besides some biasing resistors, it my be at AC ground due to a bypass capacitor from gate to ground. The lower gate is the signal input. Not all dual gate MOSFETS are cascode, though. There ae numerous applications for this device.