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For the following circuit i am trying to find the voltages at the top nodes of R2 and C2. The problem im having is that when i model it in ltspice, i dont get the voltages i have calculated. I get 0.54V for VR2 and 0.23V for VC2. The ltspice simulation gets ~600mV and ~300mV. Now the strange thing is that when i ignore the 50kohm input resistance, i get these values. Therefore i was wondering if it was possible that for this lm386 amp the input resistance was forgotten? However the strange thing is that when the cutoff frequencies are checked they get the values they should be when the 50k ohm resistance is included. (When voltages are referred to they are in amplitude)

Being the values for the answer in this question:How to find the equivalent resistances for each filter so that the cutoff frequencies can be found?

Then when i try adding the 50k Ohm resistor just to check i get closer to the output voltage (VIN) i calculated but however the cutoff freq for the lowpass filter is 90k hence i believe the input reistance is actually included in the amp.

Thus my main problem is 1. To check if the way i have calculated these output voltages with actual values are correct and if not please show me what have i done wrong and 2. What could lead to such a difference (especially when the frequency cutoffs are correct when 50k ohm resistance is included but for when 50k ohm resistance is not included for the voltages it is closer to the ltspice simulation?

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UPDATE: Is the input resistance (R_LM) in parallel with C3 or in series with C3?

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  • \$\begingroup\$ 1) Probably your LM386 model is not good enough in modelling its input impedance. Models, all of them, must be used with the greatest care because they very often "forget" something deemed "not so important" by model maker. 2)Your design is too sensitive to LM386 input impedance and this is even worst than the probable error in Spice model. Even in real life LM386 can probably vary a lot from one chip to another one. 3) So improve your design (e.g. lowering resistance values) so to have Thevenin impedance seen from LM386 input much lower than its input impedance. \$\endgroup\$
    – carloc
    May 21, 2017 at 18:32

1 Answer 1

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Since you're already adding an op amp buffer, might as well use the second out of a dual pack and make this analysis much simpler.

schematic

simulate this circuit – Schematic created using CircuitLab

Rearranging the circuit and adding the second buffer removes the dependencies between the filter and scaling. The high pass corner frequency is still dependent on the LM386 input impedance. As it it will be ~30Hz. If that's a problem, 1uF will get it down to 3Hz.

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  • \$\begingroup\$ Thankyou for the suggestion but for this task i am only allowed to use the following amp. Since part of the design brief is simplicity and a maximum number of components is only allowed for the circuit. Would you just suggest to calculate the following voltages as i have done, then use the oscilloscope to check the voltages. I was just wondering what could cause it to be different (i.e. the voltages for my calculations vs the simulations. \$\endgroup\$
    – Student
    May 21, 2017 at 17:00

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