# Amplifier Input Balancing Resistors

#### Ian787

Joined Aug 1, 2013
7
Hi there

I have a electronic schematic that I am trying to get my head around without much luck

As you can see (picture below) the circuit is a differential amplifier being fed from a 3 pin DIN connector

On the input to the amplifier are two resistors that are connected together (R101 and R102) and also joining the amplifier inputs together, I am struggling to understand the purpose of these resistors. As far as I can make out these are to balance the input to the amplifier, but this is a very strange way of doing it. Has anybody seen this configuration before and is able to explain to me how it works?

I have looked up a lot on input balancing to an op amp and can't see it being done this way anywhere at all.

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#### ericgibbs

Joined Jan 29, 2010
11,630
hi Ian,
The bottom those 2 resistors should be connected to 0V.
What signal is on the DIN input.?
E

#### Ian787

Joined Aug 1, 2013
7
hi Ian,
The bottom those 2 resistors should be connected to 0V.
What signal is on the DIN input.?
E
Hi Ericgibbs

As far as I can tell the signal coming from the DIN connectors is a DC voltage ranging from 0V - 1.4V. The voltage on the DIN connectors are coming from a diode rectifier. Before the diode rectifiers the voltage is AC in nature because it is coming from some sensors monitoring the voltage on a variable capacitor which is initially 45kV. This voltage is attenuated down until it gets to the diode rectifier before going into the circuit in the picture

I thought that the two resistor in question were for power dissipation as well as balancing

Excuse the lack of information I got this circuit given to me at work and it was very poorly documented

#### ericgibbs

Joined Jan 29, 2010
11,630
hi Ian,
As the input is from a bridge rectifier at around 1.4v, it is possible the two, 2k7 act as load for the bridge rectifier.
E

#### tindel

Joined Sep 16, 2012
756
I was thinking the same thing. Looks look like a 5.4k differential load to me.

#### Ian787

Joined Aug 1, 2013
7
Hello,

Sorry it has taken a while to get back to you on this,

Thank you for your responses, it has given me somewhere to start from with my research.

After reading the datasheet for the diode rectifiers (that was in German and needed to be translated into English) the two 2k7 resistors are indeed to load the bridge rectifier

Thank you again for your help!