Amplifier Lab problem

Discussion in 'Homework Help' started by testing12, Mar 29, 2012.

  1. testing12

    Thread Starter Member

    Jan 30, 2011
    Hello Everyone,

    I have a lab design problem for which I have to design a small signal amplifier for a gain of (positive) 50 v/v with an input resistance of 100 kOhms and and output reistance of 2.5 kOHms. I am required to do this using some sort of BJT amplifier. In addition I need to have a dynamic range of +/- 5 v. I am not limited as far as the number of stages I can use. Last note, i need to be +/- 10% of the above values.

    Here is what I have tried:
    - using two stage common emitter BJT amplifier

    problems: the gain is so great small signals cause the amp to saturate
    Also obtaining Rout = 2.5 kOhms causes the dynamic range to be so small (more like +/- 1 V instead of 5

    All suggestions are welcome, Im stuck on this.
    Thank you.
    Last edited: Mar 30, 2012
  2. testing12

    Thread Starter Member

    Jan 30, 2011
    I have revised some details above, any suggestions you can provide are appreciated.
  3. hobbyist

    AAC Fanatic!

    Aug 10, 2008
    That is a large Zinput of 100K ohms, and a small Zout of 2.5K ohms, I just designed a two stage bjt amp, using 4 transistors, the first threee transistors are direct coupled to get the Zin as high as around 120K assuming a beta min. of around 20, For worst case.

    The Zout was the original 5K ohms as your first post was written. (I made this before I seen your updated revision)

    I got the 7v. pk across a load of 20K, but the circuit is completely useless, for the requirements, because I gotr a voltage gain of around 1600, an input signal pk amplitude of 5mV. gave a 8v pk. amplitude on the output.

    It is a high gain small signal amp, using no emitter bypassing so a lot of negative feedback, but still the combination of the 3 ttransistor direct coupling gave the tremendous increase of voltage gain,

    however, the reason I am bringing all this to the table is to suggest that to get a high Z input, with bjt amp, of 100K ohms, a transistor biased as a current source could be used to substitute the bleeder resistor in the divider network normally used for biasing the base of a transistor.

    That would make the transistor be direct coupled to the next transistor, which increases gain at the same time as well. But the main thing is a higher input impedance results with that kind of setup.

    Now with all this said,

    I am only a hobbyist in this field, never had any professional experiance, All self taught, so take these suggestions as sugestions only, until the knowledgeable professionals come on board to give you real advise with your design.
    Last edited: Mar 31, 2012
  4. OscGuru

    New Member

    Apr 3, 2012
    Hi, do you still need help with this design. I would start by using emitter follower driving a ce amp with significant emitter resistance...say 30 ohms. Now you need gain and non inverting. I would have the ce stage collector drive a current mirror...pnp devices. Then you can use a 2.5 k ohm resistor on in the collector of the current mirror. Now your gain will be approx 2.5k /(30 + 1/gm) of the ce stage. This will be about 20 ohms for 1mA collector current in ce stage. The gain will be about 2.5k /50... Or..50. You will need to bias it right to get the swing on the output right. Will need very large supply voltage..only on spice I get the dynamic range. To review..the emitter first stage gives your high impedance..stage two..ce amp with significant emitter resistance...for linearity and ease of bias..this ce drives pnp current mirror to invert signal and allow 2.5 k collector resistor to get gain and output impedance right.
    Good luck.
  5. Adjuster

    Late Member

    Dec 26, 2010
    If a minimum device count is wanted, then the first stage could be a common-emitter but with a very large unbypassed emitter resistance, to give a stable but low gain (unity, maybe) with adequate input impedance. That said, if you really have to put up with very low gain devices, then as Hobbyist advises more than one device may be needed to buffer the input.

    The following stage could be another common-emitter with somewhat less emitter resistance to give the required overall voltage gain. Do not forget to allow for the transistor's internal emitter resistance (Ie/0.026, approximately) as well as the external emitter resistance.

    You might set this up as two AC coupled stages, or alternatively you might think about a circuit with a higher second stage gain, but overall feedback added maybe from the second stage collector to the first stage emitter - the latter may be better but more tricky to get right.