Op-Amp - Linear Amplifier

Op-amps can be made to do many clever things.  One application is the amplification of DC voltages.  This is useful when dealing with sensors which output in mV and it is desirable to do the processing using a wider voltage range, say, 0 - 5 volts.  The application that prompted this "lab" is the measurement of the short circuit current of a small solar panel.  This device can produce current in the range 0 - 250 mA.  This current is determined by the voltage drop across as 10 watt resistor (either 0.1 or 1.0  ohm), at low levels of irradiance, the voltage drop can be less than 10 mV.  A linear amplifier based on an op-amp with a gain set to 5 to 50 is used to create an output in the range 0 - 2.5 volts.

Reference

en.wikipedia.org/wiki/Operational_amplifier

Circuit Diagram

 The circuit diagram is shown below:

The Vcc and ground connections have been omitted for clarity.

Relationship between Vin and Vout

The Lab

A test circuit based on one of the channels of a TS 358 was setup on a breadboard.  Three combinations of R1 and R2 were tested.  The input voltage was varied using a potentiometer as a voltage divider.  For each input voltage, the output voltage was recorded.  The results are shown on the graph below:

The gradient of the line is the gain of the amplifier.  The table shows the values of R1 and R2, together with the observed and estimated gain.

R1	R2	1+R2/R1	Gradient
150k	470k	4.1	4.2
10k	150k	16.0	16.2
10k	470k	48.0	48.1

The are known to be some small errors in the meters used to measure the input and output voltages, however, the observed and predicted gains are in good agreement.

Chip Diagram

The lab was based on one channel of a TS 358 chip, this was selected for no other reason that it was the only op-amp available in the local component store on the day I cycled past.  The pin positions are shown in the diagram below: