Different Op-Amp configurations

Question 1

What is this circuit? [Picture10]

Answer 1

This is an inverting op-amp with a capacitor on the input to remove the DC offset

Question 2

What does a capacitor on the input of an op-amp do? How does it work?

Answer 2

> This removes the DC bias of the input signal.

> This is called capacitive coupling

> For AC signals the capacitor is seen as a short and for DC signals it is seen as an open circuit.

Question 3

What are the applications of using capacitive coupling on the input of an op-amp? [Picture10]

Answer 3

> Some transducers require a bias voltage in order to work

> AC signals cannot be sent into an ADC converter because the input signl must be entirely positive

Question 4

What is the problem that needs to be considered with this circuit? [Picture 10]

Answer 4

The capacitor produces a high pass filter so we need to use the following equation so we include the frequencies we want: f_3dB = 1 / (2π×C₁×R₁)

Question 5

What is the purpose of impedance matching?

Answer 5

> To provide a specific input and/or output impedance to connect two stages of circuitry.

Question 6

How can a non-inverting amplifier match impedances? What is the configuration?

Answer 6

[Image11]

> Input impedance: A non-inverting amplifier (without Rin) has a naturally very high impedance so a parallel resistor will dominate the overall input impedance. This is done with a resistor between the input and ground.

> Output impedance: R1 + R2

Question 7

How can an inverting amplifier match impedances? What is the configuration?

Answer 7

[Image12]

>Input impedance: R1 (because the + input is connected to ground)

> Output impedance: can be set by adding an output resistor.

Question 8

What is the configuration of a buffer?

Answer 8

[Picture13]

> It is an inverting amplifier with no feedback resistors

Question 9

What are the properties of a buffer?

Answer 9

> Input impedance is very high (ideally infinite)

> Output impedance is very low (ideally 0)

>Voltage gain is unity.

Question 10

Why is a buffers voltage gain unity?

Answer 10

R₂ = 0

R₁ = ∞

G = 1 + R₂ / R₁ = 1

R₂ / R₁ = 0

Question 11

What is the main application of a buffer?

Answer 11

> To prevent one stage of a circuit from loading another stage

Question 12

What is the configuration of an inverting buffer?

Answer 12

> Inverting amplifier configuration

> R1 = R2

> Input impedance is R1

Question 13

What is the summing amplifier configuration?

Answer 13

[Picture14]

> Inverting amplifier configuration

> Exploits trans-resistance behaviour

Question 14

How does a summing amplifier work?

Answer 14

> Each input contributes to an input current according to each V_in

> ∑ (V_inN / R_1N) = -V_out / R₂

> Each input can be weighted differently by selecting values of R_1N

> If each value of R_1N is equal then the circuit will add the input voltages equally.

Question 15

What is the main application of a summing amplifier?

Answer 15

Adding a DC offset to an AC output signal

Question 16

What is the configuration of an attenuation op-amp circuit?

Answer 16

[Picture 15]

> It is a potential divider circuit followed by a buffer.

Question 17

Why use an op-amp attenuation circuit?

Answer 17

> It is generally a better idea to use this approach when designing an inverting amplifier with G < 1 because the op-amp stability is much better.

Question 18

What is the op-amp single supply setup? What is the tropology?

Answer 18

[Picture16]

> -Vs = GND as this will be the lowest voltage that is available

> Input potential divider

> We need to bias the input voltage between the supply voltage and ground. Usually at half way between the two. We do this with a high-impedance potential divider.

• This ensures that the bias current is minimised.
• This also decreases the cut of frequency for the inevitable high-pass filter

Question 19

What is the reason for using a capacitor on the input?

Answer 19

> For capacitive coupling

> AC signals will be outside the voltage range because they often are negative, so the solution is to AC-couple the input signal.

Question 20

How do we select values for the C1, RA and RB?

Answer 20

> This forms a High-pass filter

> We need to include all the frequencies we need

Question 21

What is the reason for using a capacitor on the potential divider for the negative feedback?

Answer 21

If there was no capacitor it would not affect the AC gain but would cause the DC bias to be amplified so we need to ensure that the gain for the DC aspect is rolled off so saturation does not occur. We do this by adding a capacitor

Question 22

What is the reason for the capacitor at the output?

Answer 22

This is often added to remove any unwanted DC offset

Question 23

What are the different configurations for using a reference voltage with a single supply op-amp setup?

Answer 23

[Picture17] [Picture18]

Question 24

How can a supply circuit splitter be used with an op-amp?

Answer 24

[Picture19]

Question 25

What is a voltage regulator? How are they made?

Answer 25

> This is used to provide a stable reference voltage

> Made from one of the following:

• Zener diode
• Op-amp circuit used to provide a DC output
• Dedicated voltage regulator IC (best solution)

Question 26

What are single supply op-amps?

Answer 26

> Specifically designed for single supply circuits

> Differences

• The capability for the output to extend all the way to the power sipply limits especially the lower limit. This is called a rail-to-rail output.
• Rail-to-rail input

> Side effects: Internal cross-over distortion, higher output impedance etc.