Small-Signal Amp Modeling

Discussion in 'Homework Help' started by wiz0r, Dec 4, 2008.

  1. wiz0r

    Thread Starter Active Member

    May 2, 2008
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    Ok, so I got an Electronic homework. I have the following circuit(attachment #1) and I have to find the Voltage Gain, Input Resistance, Output Resistance, and then simulate the circuit using Pspice to verify my calculations.

    Well, let me tell you that everything is going not so well to put it nicely. >_>

    Ok, so let's see what I've done;

    First of all I'm doing the DC analysis(attachment #2) of the circuit to find Id of the Mosfets, and Ic1 and Ic2 of the two BJT's. So, now it's given that;

    Kn = 31.6 m A/V^2
    Vt = 1.73 V
    Lambda = 0.01 V
    Beta = 200
    Va = 100 V

    Now, Using Voltage Divisor I calculated the Voltage of R4 which is 3.75 V. Then I used KVL on the lower left loop. Assuming that Ig = 0;

    3.75 = Vgs + Id(1.8K)

    Using the mosfet Id equation;

    Id = 31.6m(Vgs - 1.73)^2

    Now, putting those equation together I found out that;

    Vgs = 1.53 V || 1.93 V, Using 1.93 V I found out that;

    Id = 1.024m A

    Using KVL in the middle upper loop;

    (3.3k)Ie1 + 0.7 - (4k)Id = 0

    Now, I get that;

    Ie1 = 1.029 m A, therefore Ic1 = 1.024 m A

    Now, using KVL in the lower right loop, assuming that Ib is really small(the prof gave me this tip, so yeah..)

    Ie2(7k) + 0.7 - Ic1(8k) = 0

    Ie2 = 1.07m A, Ic2 = 1.065 m A

    Ok, that's everything. Comparing those values with the values I get from PSpice(attachment #3), they are absolutely wrong. So, can anyone point me out what am I doing wrong? Or maybe I'm doing something wrong with Pspice? ;<

    I don't know. If it helps I'm using Pspice 9.1 student edition.

    So please, help me. Any help is greatly appreciated,

    PS: There is no given AC Voltage, but as a tip of the professor, he told us to use 1V, because he said that when we plotted the output voltage we could easily see the voltage gain.

    Edwin
     
    Last edited: Dec 4, 2008
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    First amp - it seems counter intuitive to have the FET's like that. They are drawn as Power FET's, not as JFET's, and one might expect a differential stage at the input. There is also no feedback. It appears odd to use a driver transistor that can handle more power than the output transistor.

    Most of that applies to the DC amp, except that it has no input in addition to the above.
     
  3. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Are you using MOS and BJT with the same parameters as in the exercise in Spice? If not, this is the reason if your calculations are correct.
     
  4. wiz0r

    Thread Starter Active Member

    May 2, 2008
    64
    0
    Mm, I'm a bit confused. First of all they are all the same Amp. My first attachment is the problem given by the professor, the second is just me doing the DC analysis of the circuit, and the third is the same amp with the voltages and currents. Also, there are no JFET's in my amp. x_o

    And, well, yes, I think I edited the transistors to have the same parameters given by the problem. I fixed Beta and VA in the BJT's, and left everything as it was before. And for the Mosfets I calculated a Kp from the Kn given. I made the L = 10u, and the W = 1u, therefore Kp = 0.00632, also lambda = 0.01, Vto = 1.73. So, yeah, I think I got that covered..

    PS: There is no given AC Voltage, but as a tip of the professor, he told us to use 1V, because he said that when we plotted the output voltage we could easily see the voltage gain.

    Edwin
     
    Last edited: Dec 4, 2008
  5. The Electrician

    AAC Fanatic!

    Oct 9, 2007
    2,281
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    Can you give a value for the transconductance of your FETs?
     
  6. mik3

    Senior Member

    Feb 4, 2008
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    Are they MOSFETs or JFETs?
     
  7. wiz0r

    Thread Starter Active Member

    May 2, 2008
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    Can you give a value for the transconductance of your FETs?

    Well, assuming that the Id that I calculated is correct, well since;

    gm = sqrt(4*Kn*Id)
    gm = sqrt(4 * 31.6m * 1.024m)
    gm = 0.011377

    Also, they are MOSFET's.
     
  8. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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    It looks like R1 is a load resistor. Do you want the output resistance looking to the left into C4 without R1 in place? Or, is R1 to be included?

    What frequency of operation are you supposed to use? And, if your instructor didn't give you one, what did you use in your spice simulation?
     
  9. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    Your calculations are wrong because you assumed that M5 conducts also 1.024mA. This is wrong because yo don't know exactly what is the Vgs of M5. Thus it may not be able to conduct 1.024mA with the Vgs developed for it. You have to combine the effects of both MOS to find the current. Maybe if you use transistors models the analysis will be easier. Its up to you.
     
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