Preamp Circuit Help

Discussion in 'The Projects Forum' started by threefaye, Jan 4, 2010.

  1. threefaye

    Thread Starter New Member

    Jan 4, 2010
    3
    0
    Hello Everyone,

    I have been finding this website helpful for a long time but this is my first time posting here, so apologies in advance for possibly posting in the wrong section.
    I have this preamp circuit and I have a question about it, the nodes 1 2 is a current source and the circuit seems to me to be a simple IV converter with 1G gain (1G resistor). However, it has these inputs to the opamp from an obscure set of circuitry that I am having difficulty making heads and tails of. I am suspecting that they are some kind of impedance converter but I am not sure.
    Any suggestions or advice as to the function of this circuit or resources I should look up would be very welcome.

    Thanks.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
  3. threefaye

    Thread Starter New Member

    Jan 4, 2010
    3
    0
    Thanks, I figured that out, but I am not really sure what purpose it serves.
     
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    Perhaps the elaborate discrete component front end electronics is intended to add improved high input impedance / high common mode rejection. You might add low drift and low offset - who knows. Anyway, someone has gone to a lot of trouble so there would have to be good reasons for doing this. What's the purpose / application of the circuit?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The GAA3R512 is a 0.35pF to 3.5pF trimmer cap.

    Usually caps that small aren't used unless you're working with RF.
     
  6. threefaye

    Thread Starter New Member

    Jan 4, 2010
    3
    0
    This circuit is used to amplify the tunneling current signal from an STM microscope and convert it to a voltage signal, the tunneling current is very small (~10pA), hence the need for a 1G resistor, the bandwidth required is around 10Khz, therefore a regular opamp usually won't do the trick. However, I don't really see the function of this circuit.
     
  7. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    It's an interesting circuit .....

    The front end looks like a long-tailed pair arrangement (2N3904's) with a cascode input setup - the U430 matched pair each have the 2N2904 as a load. The 2N3904 bases are AC grounded as one would expect in a cascode setup.

    The 2N4401 setup looks like a current source to feed the "tail".
     
Loading...