Feb 28: An AC Microphone Amplifier

AC Microphone Amplifier

Use of "Single Supply" Op Amp
-can be operated with V+ = 12 V, V- = -12 V but also can be operated with V- = GND (and output can swing all the way to V-)
-Though we don't take advantage of the single supply's "hallmark," single supply op amps allows one to work down to its negative supply (ground) 

Goal:
-Build an audio amplifier circuit
-Amplify the output of a microphone
-Drive a speaker (actually a piezoelectric buzzer)

Signal of Interest: 
-Electrical output of a microphone (with a small amplitude of 20 mV or less and is AC i.e. audio frequencies ranging from 20 Hz - 20 MHz)


1) Setting the Gain of the Audio Amplifier (Circuit #1)

Note: Configured the '358 as a non-inverting amplifier

Why? 

Here, a non-inverting amplifier has a low output impedance (for small signals) while the input impedance is very high, which is useful when an application requires such high input impedance

-Choosing components and values to create CKT1 to achieve reasonable gain

-Assumptions:

Audio signal of interest ~ 15 mV

Output voltage in range 0-3.5 V (since powering amplifer with +5 V and '358 can go from ground or close to ground to about 1.5 V below the positive supply)

First, we thought about the effective resistance we'd need for CKT 1, which we calculated by using the equation for non-inverting amplifier:

Vout = 3.5 V

Vin = 15 mV = 0.015 V

So: 3.5 V = (1 + (1 M/R_ckt1))* 0.015 V

R_ckt1 = 4303 Ohms or about 4.3 kOhms

To check/review, we used the Golden Rule #1, which says:

Vin = V- = 0.015 V = (R_ckt1/(R_ckt1 + 1 M))*3.5 V

so R_ckt1 = 4.3 kOhms

Then, we thought about how to have a range of output voltage from 0-3.5 V and thought we could use a potentiometer. To get the right effective resistance, we needed a 10 k pot and R1 and R2 values that produced an effective resistance of 4.3 kOhms.

Our initial idea of CKT1 to get the right effective resistance AND to have variable resistance to have a range of output voltage values

However, we soon realized that we had forgotten to consider gain (our input voltage is only about 15 mV so we needed to amplify the signal).

Thus, we took the ratio of the amplitude of output voltage over input voltage = (3.5/2) / 0.015 = 100

and decided we wanted a gain of about 100 (up to 100 or else above 3.5 V)

Then, using the gain factor for non-inverting amplifiers, we determined R_ckt1 again:

1 + (1 M / Rckt_1) = 100

Rckt_1 approx = 10 kOhms

We tested combinations of resistors and potentiometers that satisfied this R_ckt1 requirement and decided on:

R_pot = 10 k

R2 = 10 k 

And got rid of R1

so now:

because R_pot can be adjusted from 0 to 10k which means our range of resistance would be from 10 k to 20 k (G=100 to G<100 satisfying our upper limit for gain)

*Later changed R2 = 6.8 k because our actual gain wasn't high enough (so output signal not large enough)  (Mentioned in detail later)

We later realized we took care of volume control by adding the potentiometer.

And by considering the max gain to be 100 and using appropriate components and values to upkeep that requirement, we were able to keep the user from making the output rail.

Clipped signal

Added buzzer to test

distorted signal after adding buzzer

To keep the signal from getting clipped below ground (have room for voltage swings), we had to make the audio signal ride on a DC level. We ultimately wanted the DC offset to pass through unaffected (G = 1) while the AC signal would have a large gain (G= 100), i.e. we wanted CKT1 to be a follower at DC but an amplifier at AC.

To achieve this, we added a capacitor to the circuit. At high frequency, C would be just like a wire, so what we had to worry about when choosing the capacitance was the lower frequency limit of 20 Hz. 

So now, considering the effective resistance:

R_ckt1 = 1/(omega*C) + R2 + R_pot = 10k

1/(2*pi*20 Hz)  = 1/(omega*C) since we had to consider the low f case.

C = 1.2 uF but we used 4.7 uF and R2 = 6.8 kOhms to increase the gain (our output signal was not strong enough)

Adding a Microphone

-replacing function generator with microphone to encode voice

-microphone includes a high input impedance FET

During silence, Vout = 5V because no current flow in circuit

Video of us looking at our voices on the scope

Connecting Microphone to Amplifier

-Microphone a +5V DC signal with a small (~20 mV) AC signal on top so need an intermediate circuit between the microphone and amplifier (CKT#2)

CKT#2 Requirements:

1) block large DC voltage from microphone but pass AC signal

2) make AC audio signal ride on a ~1.75 V DC offset so that the audio signal lies in the middle of the output range of amplifier

-want Zin of second subcircuit to be much larger than Zout of previous stage and also not waste power

Our idea for CKT2:

We want:

Zout,micro << Zin, ckt2

-so we want Zin, ckt2 = R1 || R2 to be relatively large

and 

Zout, ckt2 << Zin, op amp 

where Zout, ckt2 = R1 || R2

-The second condition should not be difficult since Zin, op amp is huge (~10^12)

Also, since we want DC offset of 1.75 V, the ratio of R1 and R2 have to be such that 

1.7 = (R2/(R1+R2))* 5 

so we picked: R1 = 2.7 MOhms and R2 = 1.0 MOhms 

-This satisfied our R1 || R2 requirement (relatively large and the right ratio)

Above, we effectively created a high pass filter so our worst case was 20 Hz (low f limit). So we wanted f-3db << 20 Hz. And if R1 and R2 are too small, we'd waste power.

And since we want f-3db << 20 Hz or 1/(RC) << 20 Hz (lower f limit in our case),

C has to be less than 2.5E-5 F so we picked a capacitor with capacitance in the uF. 

So our circuit looked like: