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PNP Cascode

  1. Oct 11, 2015 #1

    Zondrina

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    Hi everyone. I have seen the design for an NPN Cascode many times, but I want to build a PNP Cascode. I have scoured the internet for a standard design, but to no avail.

    The image below is that of a NPN Cascode. I want to create a PNP cascode by flipping things around. I have placed a red line through the circuitry. On the left side of the red line I expect nothing to change when building the PNP Cascode. On the right side of the red line is where I expect things to change.

    Screen Shot 2015-10-11 at 10.58.45 AM.png

    Here is what I am thinking:

    1. I will turn transistors ##Q_1## and ##Q_2## into PNP transistors.

    2. In the emitter of ##Q_2## there will be no resistor, only the power supply connection ##V_{CC}##.

    3. Between the collector of ##Q_2## and the emitter of ##Q_1##, I will place ##R_C## just before the loading circuit. Then I will place the loading circuit.

    4. Just after the loading circuit, I will place ##R_{E1}##, ##R_{E2}## and ##C_E##.

    5. The collector of ##Q_1## will then be wired to ground.

    Does this sound reasonable?

    If not, does anyone know what the standard design would look like and could show me?

    Thank you.
     
  2. jcsd
  3. Oct 11, 2015 #2

    Zondrina

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    I took the time to make a quick blackboard sketch of what I thought it looks like:

    IMG_0963.JPG

    Seem reasonable?
     
  4. Oct 11, 2015 #3
    Your PNP version is wrong, and will not work. The Cascode is CE stage + CB stage.
    The correct circuit look like this
     

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  5. Oct 11, 2015 #4

    Zondrina

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    It was just hitting me and I came back and saw your post.

    I drew this:

    https://gyazo.com/d3cfd6526d94f4ce09a6ab1885debf01

    It looks pretty much like yours, except the capacitor ##C_E## should be grounded.
     
  6. Oct 12, 2015 #5
    Almost right. R3 resistor must be connected to VCC and R1 to GND.
    To convert any NPN circuit into PNP version simply amply this rules:
    1 - Change transistor type NPN into PNP
    2 - NPN ground now becomes PNP VCC
    3 - NPN VCC change to PNP GND (simply reverse the power supply polarity).
    4 - If necessary reverse electrolytic capacitor polarity
    But this change is not necessary (Vcc is also a good place).
     
    Last edited: Oct 12, 2015
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