jrnb3 said:
for the input, the mic has an output resistance of 5k, which is in parallel with 10k, and in series with 2.2k. combined with the 4.7u leads to frequency of 6.1Hz.
Yes. the frequency response is down to about 6.1Hz.
The 470u cap keeps a steady supply voltage for the mic and preamp, due to fluctuations in the internal resistance of the battery.
No. The
signal causes fluctuations in the supply voltage (when the internal resistance of the battery increases as it runs down) if the 470uF capacitor is not there to smooth the fluctuations.
It is unclear is what frequency we are trying to amplify (what is the typical frequency of a heartbeat?)
A heartbeat is a DC pressure wave. A microphone cannot produce DC so the circuit responds to very low frequencies near DC.
unclear is what the role of the 1k is
The 1k resistor and the 470uF capacitor are a filter.
what voltage levels are expected at the input of the preamp.
Maybe the input to the circuit is 10mV peak.
For the preamp, .... The input resistance is 10E12
The input resistance of the preamp is 2.2k ohms (R2) which is too low but it works.
Yes, it inverts with a gain of 8.5.
unclear how to figure what frequency is being amplified
U1A amplifies frequencies from 6.1Hz to 100kHz.
and what the output resistance is
The output resistance of an opamp is "zero ohms". It is actually the physical 75 ohms divided by the open loop gain of about 200,000, times the gain. So it is actually 0.003 ohms at very low frequencies.
For the low pass filter, also a TL072. function is to filter higher frequency noise. unclear where the noise comes from-- only lower frequencies have been amplified.
The microphone picks up sounds of road traffic outside and people talking nearby. The first opamp amplifies it.
cutoff frequency is 103Hz (what is in this frequency range?). it is a 2nd order butterworth filter, meaning it has a flat amplitude response. its gain at 103Hz is .707, leading to an overall gain of about 1.6.
Yes, the response of the Butterworth filter cutsoff at 103Hz.
unclear what the 33k and 56k are for
The gain of the opamp is (33k/56k) + 1= 1.59. If one filter capacitor is twice the value of the other then the gain can be 1 and it will still be a Butterworth filter.
what the output resistance is.
The output resistance of an opamp is "zero ohms". It is actually the physical 75 ohms divided by the open loop gain of about 200,000, times the gain. So it is actually 0.0006 ohms at very low frequencies.
The volume control is a variable resistor, which when increased, increases the input bias which results in higher output voltage
No. The bias voltage is not changed by the volume control. It changes the signal amplitude due to its adjustable voltage divider action.
unclear what the limit is for this resistor value.
The value of the volume control can be from 2k (the lowest value an opamp can drive) to about 50k which is the input resistance of the LM386 amplifier.
The output amp is a LM386. its input resistance is 50k in series with the pot. it has a very low load resistance and is able to power low impedance headphones. although this amp is better, i will be using a 741 amp because i have to do a simulation is PSPICE, and i dont know how to create a subcircuit. The 741 opamp has an input resistance of 2Meg, unclear of the output resistance.
A 741 is an opamp. An LM386 is a power amp which is completely different.
A 741 can be loaded with no less than about 2k ohms. An LM386 is made to have a load down to 8 ohms.
A 741 needs to be biased correctly and needs to have two resistors to set its gain.
An LM386 is internally biased and has its gain internally set at 10.
A 741 doesn't work if its inputs are at ground (0V).
An LM386 is designed for its inputs to be at ground.
unclear what the role of 3.9 and .1u are
They are the load on the LM386 at very high frequencies where the speaker's impedance becomes very high due to its inductance. The LM386 will oscillate at a very high frequency without them.
about to put circuit in PSPICE wish me luck.
Good luck. ;D
