# Learning to read circuit designs. Need help.

#### whoknowsmerida

Joined Jun 26, 2018
4
I'm new learning to read circuits. so please bear with me as i ask some basics. Here is the circuit from a document i am reading about Microphones and pre-amps.

The actual circuit is from a document listed below.

http://www.ti.com/lit/ug/tidu765/tidu765.pdf

I'm not sure how the circuitry in the pre-amp actually works. I'll write my questions one by one so i can better understand.

1. why is the direction of the Current (Imic, Idc, Iac ) pointing towards the Electret Microphone? I thought the current should flow out into the next stage , towards the preamp ?

2. What is the significance of a resistor/capacitor combination across the circuit? Why not only have resistors to control the flow of current ?

3. The document states "the gain of the pre-amp is determined by R2 but the noise gain of the op amp is determined by the ratio of R2 to R1" . i don't understand how the Resistance R2 can control the gain of the pre-amp.

4. What is the significance of the module at the '+ve' end of the Op-amp U1.

5. What book would one recommend to get started with circuit designing. ?

Thank you.

#### WBahn

Joined Mar 31, 2012
30,055
I'm new learning to read circuits. so please bear with me as i ask some basics. Here is the circuit from a document i am reading about Microphones and pre-amps.

View attachment 169015

The actual circuit is from a document listed below.

http://www.ti.com/lit/ug/tidu765/tidu765.pdf

I'm not sure how the circuitry in the pre-amp actually works. I'll write my questions one by one so i can better understand.

1. why is the direction of the Current (Imic, Idc, Iac ) pointing towards the Electret Microphone? I thought the current should flow out into the next stage , towards the preamp ?
Because the person that drew the circuit chose that direction. They probably want to discuss the circuit from the context of the current flowing in the microphone and, when there is no audio signal, the microphone current is the same as Idc.

2. What is the significance of a resistor/capacitor combination across the circuit? Why not only have resistors to control the flow of current ?
Depends on which components you are referring to. C3 is a coupling capacitor to let the AC signal through to the preamp while blocking the DC bias signal. In conjunction with the resistors it sets the lower cutoff frequency for the preamp. C2 kills the gain at higher frequencies, so it (again, in conjunction with the resistors) sets the upper cutoff frequency. C5 only allows the AC component of the signal to reach the output and C6 is there to stabilize the bias reference voltage.

3. The document states "the gain of the pre-amp is determined by R2 but the noise gain of the op amp is determined by the ratio of R2 to R1" . i don't understand how the Resistance R2 can control the gain of the pre-amp.
Then you need to learn the basics of opamp circuits. The noninverting input is held at a fixed bias voltage and so a given change in the microphone current results in a given signal current through C3. Within the passband, C2 looks pretty much like an open circuit, so that current has to go through R2. Increase R2 and you increase the voltage developed across it.

4. What is the significance of the module at the '+ve' end of the Op-amp U1.
Assuming you are talking about the part of the circuit that generates Vb, that sets the bias or reference voltage for the opamp so that the output can go above and below it. This is important because it is a single-supply opamp and so it needs to be biased about midway.

5. What book would one recommend to get started with circuit designing. ?
Depends on a lot of things. What your background is, what your goals are, what your math level is like, whether you really want to understand and design circuits from an engineering perspective, or just learn to put building blocks together in pretty rote ways and learn how to tweak them to get desired results.

#### Audioguru

Joined Dec 20, 2007
11,248
The DC arrows show Idc which is the DC current powering the Jfet in the electret mic.
I agree with you that Iac should point towards the opamp.
Your other questions show that you need to learn the simple basics about opamps.

#### WBahn

Joined Mar 31, 2012
30,055
The DC arrows show Idc which is the DC current powering the Jfet in the electret mic.
I agree with you that Iac should point towards the opamp.
Your other questions show that you need to learn the simple basics about opamps.
One advantage of Iac as shown is that a positive Iac results in a positive output voltage excursion.

#### Audioguru

Joined Dec 20, 2007
11,248
One advantage of Iac as shown is that a positive Iac results in a positive output voltage excursion.
No. The AC signal current alternates positive and negative. Then the output signal also alternates positive and negative.

#### MrChips

Joined Oct 2, 2009
30,805
A TI design and test board for an electret mic preamp?

Things must have been slow at TI in 2015 or John Caldwell was bored and looking for something to do.
Just ignore that drawing.

#### whoknowsmerida

Joined Jun 26, 2018
4
Because the person that drew the circuit chose that direction. They probably want to discuss the circuit from the context of the current flowing in the microphone and, when there is no audio signal, the microphone current is the same as Idc.

Depends on which components you are referring to. C3 is a coupling capacitor to let the AC signal through to the preamp while blocking the DC bias signal. In conjunction with the resistors it sets the lower cutoff frequency for the preamp. C2 kills the gain at higher frequencies, so it (again, in conjunction with the resistors) sets the upper cutoff frequency. C5 only allows the AC component of the signal to reach the output and C6 is there to stabilize the bias reference voltage.

Then you need to learn the basics of opamp circuits. The noninverting input is held at a fixed bias voltage and so a given change in the microphone current results in a given signal current through C3. Within the passband, C2 looks pretty much like an open circuit, so that current has to go through R2. Increase R2 and you increase the voltage developed across it.

Assuming you are talking about the part of the circuit that generates Vb, that sets the bias or reference voltage for the opamp so that the output can go above and below it. This is important because it is a single-supply opamp and so it needs to be biased about midway.

Depends on a lot of things. What your background is, what your goals are, what your math level is like, whether you really want to understand and design circuits from an engineering perspective, or just learn to put building blocks together in pretty rote ways and learn how to tweak them to get desired results.
I think i have to learn/read more about some basics of op-amps and circuits in general. Thank you for replying. it will help me get started.

#### KeepItSimpleStupid

Joined Mar 4, 2014
5,088
The circuit means you need to know:
1. A little about OP amps
2. The formula f=1/(2*PI*R*C)
3. Single supply OP amps with AC signals
4. Electret microphones which need power.
5. C2 and R2 are probably stability or bandwidth limiters.

Your only interested in AC signals, but there is no negative supply so you have to bias the output usually to mid-supply.
the RC combinations change the gain depending on frequency. That formula relates to a single-pole filter where the gain is down about 70.7%
capacitors also couple AC signals. The decoupling caps for the OP amp are not shown. Usually ceramic capacitors on the power pins close to the IC.

#### whoknowsmerida

Joined Jun 26, 2018
4
The circuit means you need to know:
1. A little about OP amps
2. The formula f=1/(2*PI*R*C)
3. Single supply OP amps with AC signals
4. Electret microphones which need power.
5. C2 and R2 are probably stability or bandwidth limiters.

Your only interested in AC signals, but there is no negative supply so you have to bias the output usually to mid-supply.
the RC combinations change the gain depending on frequency. That formula relates to a single-pole filter where the gain is down about 70.7%
capacitors also couple AC signals. The decoupling caps for the OP amp are not shown. Usually ceramic capacitors on the power pins close to the IC.

Will do. thank you. I've already started on op-amps .

#### WBahn

Joined Mar 31, 2012
30,055
No. The AC signal current alternates positive and negative. Then the output signal also alternates positive and negative.
I never said that the AC signal didn't alternate or that it was never negative. I said that a positive Iac results in a positive output voltage excursion. In other words, the two are in phase. This allows the circuit to be discussed without having to deal with the fact that the opamp configuration is intrinsically an inverting one -- you simply incorporate the inversion into the polarity assigned to Iac.