Op Amp Applications

1EEE 3308

Analyses for A∞ and loop gain are separate.

Design breaks down into two parts:

- Arranging A∞ to do the job- Ensuring the feedback loop has enough loop gain and is stable.

Here we focus on the A∞ part, assuming youalready know how to get a stable feedback loop.

Non-Inverting Amp

2EEE 3308

R2R1

vsvo

Rout

Rin

Rin→ ∞

Rout→ 0

Ideal forVoltage Amp

A∞ =1+

R2

R1

Non-Inverting Amp

3EEE 3308

R2R1

vsvo

Rout

Rin

Rin→ ∞

Ideal forVoltage Amp

Voltage Followervs

vo

Rout

Rin

A∞ =1

Rin→ ∞

Rout→ 0

Ideal VoltageBuffer

Inverting Amp with R1 → ∞

A∞ =1+

R2

R1

Rout→ 0

Voltage Follower

4EEE 3308

vsvo

Rout

Rin

A∞ =1

Rin→ ∞

Rout→ 0

Ideal VoltageBuffer

In what sense is this “ideal”?

Voltage Follower

5EEE 3308

vsvo

Rout

Rin

A∞ =1

Rin→ ∞

Rout→ 0

Ideal VoltageBuffer

RF

vsvo

RL

RS

Adding RS, RF or RL

reduces T but has no effect on A∞.

Still A∞ = 1.

In what sense is this “ideal”?

Inverting Amp

6EEE 3308

R2

vo

Rout

Rin

A∞ =−

R2

R1

Rin→ R1, which is not ideal.

Rout→ 0vs

R1

Ideal forVoltage Amp

Inverting Amp

7EEE 3308

R2

vo

Rout

Rin

A∞ =−

R2

R1

Rin→ R1, which is not ideal,

Rout→ 0 Ideal for

Voltage Ampvs

R1

so including Rs ≠ 0 reduces the gain.

R2

vo

vs

R1RS

A

v≅−

R2

R1 +RS

Inverting Amp

8EEE 3308

R2

vo

Rout

Rin

A∞ =−

R2

R1

Rout→ 0 Ideal for

Voltage Ampvs

R1

RB

R2

vo

vs

R1

RC

RA

On the other hand,adding RA, RB or RC

reduces T but hasno effect on A∞ = -R2/R1

Summing Amp

9EEE 3308

R2

vo v

o≅−R2 ⋅

v1

R1

+v2

R1

+L +vn

Rn

⎛

⎝⎜⎞

⎠⎟

R1v1

R2v2

vn

Rn

.

..

T decreases with n:

I to V Converter/ Transimpedance Amp

10EEE 3308

R2

vo

Rout

Rin

A∞ =−R2

is Rin→ 0

Rout→ 0

Ideal forTransimpedanceAmp (iin, vout)

Current-Mode Outputs:Put load in series with the

feedback signal path

11EEE 3308

R2R1

vs

io

Rout

V to I Converter/Transadmittance Amp

12EEE 3308

R2R1

vs

io

Rout

iout

vs

≅A∞ =1R1

Rin→ ∞

Rout→ ∞

Ideal forTransadmittanceAmp (vin, iout)

Rin

V to I Converter/Transadmittance Amp

13EEE 3308

R2R1

vs

ioRA, RB, RC, and RD

reduce T but do notaffect A∞ = 1/R1

RA

RC

RB

RD

Current Buffer

14EEE 3308

R2

Rout

A∞ =1

is

io

Rin

Rin→ ∞

Rout→ ∞

Ideal forcurrent incurrent out

Current Amplifier

15EEE 3308

RL

R3

io

R2is

Current Amplifier

16EEE 3308

RL

R3

io

R2is

v3 v3=(0) −isR2virtual ground

Current Amplifier

17EEE 3308

RL

R3

io

R2is

v3

i3

Current Amplifier

18EEE 3308

RL

R3

io

R2is

v3

v3=(0) −isR2

i3 =v3

R3

=−R2

R3

is

io =is −i3 =is − −R2

R3

is⎛

⎝⎜⎞

⎠⎟

i3

Current Amplifier

19EEE 3308

RL

R3

io

R2is

v3

v3=(0) −isR2

i3 =v3

R3

=−R2

R3

is

io =is −i3 =is − −R2

R3

is⎛

⎝⎜⎞

⎠⎟

A∞ =1+

R2

R3

i3

Current Amplifier

20EEE 3308

RL

R3

io

R2is

A∞ =1+

R2

R3

Current Amplifier

21EEE 3308

RL

R3

io

R2

is

RA

RSRC

RB

RO

Adding RA, RB, RC, RS, or RO

reduces T but has no effect on A∞

Current-Mode Outputs:Put load in series with the

feedback signal path

22EEE 3308

RLoadR1

vs

io

Later we’ll see that there is a differentway to take current-mode outputs from a feedback circuit using transistors.