Multiple Feedback Bandpass Filter: Simulation question.

Discussion in 'General Electronics Chat' started by protobit, Dec 26, 2012.

  1. protobit

    Thread Starter New Member

    Dec 18, 2012
    2
    0
    I'm trying to design a RF receiver to pick up AM 60khz and what I'm thinking is to make a two-stage BJT amplifiers and an active opamp filter.

    For the active op-amp filter I'm trying to use a multiple feedback bandpass filter with middle frequency of 60khz, middle band gain of 1 and bandwidth of 5khz.

    To solve for the component values I'm using matlab:
    Code ( (Unknown Language)):
    1. %Multi-Feedback BandPass OpAmp Filter
    2. syms R1 R2 R3
    3. C1 = 1e-9;
    4. w_m = (1/(C1^2*R3) * (1/R1+1/R2))^0.5;
    5. A_m = R3/(2*R1);
    6. B = 1/(pi*C1*R3);
    7. % Q = 0.5*(1+R1/R2)^0.5*(R3/R1)^0.5;
    8.  
    9. solu = solve(w_m == 2*pi*60e3, A_m == 1, B == 5e3);
    10.  
    11. sol_R1 = double(solu.R1);
    12. sol_R2 = double(solu.R2);
    13. sol_R3 = double(solu.R3);
    14.  
    15. fprintf('\nC1 = %.3e\nR1 = %.3e\nR2 = %.3e\nR3 = %.3e\n\n', C1, sol_R1, sol_R2, sol_R3)
    Results:
    Code ( (Unknown Language)):
    1. C1 = 1.000e-09
    2. R1 = 3.183e+04
    3. R2 = 1.109e+02
    4. R3 = 6.366e+04
    Which produces the expected Bode plot with the following matlab code:
    Code ( (Unknown Language)):
    1. R1 = sol_R1;
    2. R2 = sol_R2;
    3. R3 = sol_R3;
    4. C1 = 1e-9;
    5.  
    6. num = [1/(R1*C1), 0];
    7. den = [1,...
    8.         2/(R3*C1),...
    9.         1/(C1^2*R3)*(1/R1+1/R2)];
    10.  
    11. h = tf(num, den);
    12. bode(h);
    Using these results I am simulating the circuit using LTSpice:
    [​IMG]
    Netlist:
    Code ( (Unknown Language)):
    1. XU1 0 N003 pos neg V_out LM741
    2. V1 pos 0 9
    3. V2 0 neg 9
    4. V3 N001 0 AC 1
    5. R1 N002 N001 31.8k
    6. C2 N003 N002 1n
    7. C1 N002 V_out 1n
    8. R3 N003 V_out 63.66k
    9. R2 N002 0 110
    10. .lib lm741.sub
    11. ;tran 0 10m 0 .01
    12. .ac oct 1000 1k 1MEG
    13. .backanno
    14. .end
    Which produces a frequency response centered at 40kHz (not 60kHz):
    [​IMG]

    I understand that the matlab and SPICE wont match usually because one is ideal while the other is not. My question is why is their such a discrepitancy between the designed middle frequency of 60khz and the simulated middle frequency of 40khz. How can I modify and/or model my circuit to work for 60khz?
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    A lousy old 741 opamp has trouble above only 9kHz. It cannot go as high as 60kHz and I am surprised to see 40kHz.
    Most opamps can go up to 100kHz. Try one.
     
  3. protobit

    Thread Starter New Member

    Dec 18, 2012
    2
    0
    So I'm somewhat new to electronics. I just simualted a RC4558 and the center frequency is now 50kHz. What would be a good suggestion for a better opamp? Also what specs on a datasheet would be telling of this?

    Edit: This would be slew rate correct?
     
    Last edited: Dec 26, 2012
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    A 741 is about 45 years old. A 4558 is about 42 years old. Use a much better newer opamp.

    The Gain Bandwidth Product and the slew rate should be very high for a peak at 60kHz.
     
    protobit likes this.
  5. TheGuy

    New Member

    Dec 27, 2012
    2
    0
    Just curious, what kind of op amp would you recommend for something like this?
     
  6. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    I am an audio guy. I use low noise 100kHz audio opamps. You need something with a very wide bandwidth.
     
  7. TheGuy

    New Member

    Dec 27, 2012
    2
    0
    Right, so what kind of models specifically? Is there a particular line that you prefer over others? What kind of values would be preferable for something like this?
     
  8. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Is the bandpass what you want? Its peak is extremely narrow but its skirts are very gradual and wide. The opposite of a "normal" radio.
     
  9. Veracohr

    Well-Known Member

    Jan 3, 2011
    551
    76
    Gain Bandwidth Product is a good one to look for. Go for higher. Also slew rate (depending on desired output level).
     
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