TS616

Intermodulation distortion product

17/37

5

Intermodulation distortion product

The non-ideal output of the amplifier can be described by the following series, due to a non-

linearity in the input-output amplitude transfer:

A one-frequency (one-tone) input signal contributes to a harmonic distortion. A two-tone

input signal contributes to a harmonic distortion and an intermodulation product.

This intermodulation product, or rather, the study of the intermodulation distortion of a two-

tone input signal is the first step in characterizing the amplifiers capability for driving multi-

tone signals.

The two-tone input is equal to:

giving:

In this expression, we can extract distortion terms and intermodulation terms from a single

sine wave: second-order intermodulation terms IM2 by the frequencies (

(2

We can measure the intermodulation product of the driver by using the driver as a mixer via

a summing amplifier configuration. In doing this, the non-linearity problem of an external

mixing device is avoided.

Figure 45.

Non-inverting summing amplifier for intermodulation measurements

V

out

C

0

C

1

V

in

C

2

V

in

2

C

n

V

in

n

++

+

=

V

in

A

ω

sin

B

ω

sin

+

=

t

C

0

C

ω

sin

B

ω

sin

+

() C

ω

sin

B

ω

sin

+

()

ω

sin

B

ω

sin

+

()

n

++

+

=

Vout diff.

300

Ω

910

Ω

+Vcc

100

Ω

50

Ω

49.9

Ω

49.9

Ω

1/2

TS616

-Vcc

√2:1

1/2

TS616

910

Ω

300

Ω

Rout1

Rout2

1k

Ω

1k

Ω

1k

Ω

1k

Ω

49.9

Ω

49.9

Ω

49.9

Ω

49.9

Ω

50

Ω

100

Ω

1:

√2

50

Ω

100

Ω

1:

√2

Vin1

Vin1

_

+

_

+

Vout diff.

300

Ω

910

Ω

+Vcc

100

Ω

50

Ω

49.9

Ω

49.9

Ω

1/2

TS616

-Vcc

√2:1

1/2

TS616

910

Ω

300

Ω

Rout1

Rout2

1k

Ω

1k

Ω

1k

Ω

1k

Ω

49.9

Ω

49.9

Ω

49.9

Ω

49.9

Ω

50

Ω

100

Ω

1:

√2

50

Ω

100

Ω

1:

√2

Vin1

Vin1

_

+

_

+