REV.

–8–

SSM2019

BUS SUMMING AMPLIFIER

In addition to its use as a microphone preamplifier, the SSM2019

can be used as a very low noise summing amplifier. Such a circuit

is particularly useful when many medium impedance outputs

are summed together to produce a high effective noise gain.

The principle of the summing amplifier is to ground the SSM2019

inputs. Under these conditions, Pins 1 and 8 are ac virtual grounds

sitting about 0.55 V below ground. To remove the 0.55 V offset,

the circuit of Figure 5 is recommended.

A2 forms a “servo” amplifier feeding the SSM2019 inputs. This

places Pins l and 8 at a true dc virtual ground. R4 in conjunction

with C2 removes the voltage noise of A2, and in fact just about

any operational amplifier will work well here since it is removed

from the signal path. If the dc offset at Pins l and 8 is not too

critical, then the servo loop can be replaced by the diode biasing

scheme of Figure 5. If ac coupling is used throughout, then Pins 2

and 3 may be directly grounded.

–

IN

SSM2019

R4

5.1k

R3

33k

C1

0.33 F

R2

6.2k

C2

200 F

+

IN

A2

R5

10k

V

IN4148

TO PINS

2 AND 3

Figure 5. Bus Summing Amplifier

A