Op Amp - Dual Rail Supply

This configuration is a Inverting Operational Amplifier using a dual-rail supply consists of two 9 volt batteries. The supply provides +9V (one rail), with respect to the circuit common(ground) as well as -9V (another rail).
The resistors value for R1 and R2 are 10K and the value for R3 is 100K.
Notice that one input port, feeds into R2. The other end of R2 and R3 form a voltage divider circuit around the inverting input pin, with R3 being the feedback circuit. The IC's inverting voltage is a sum of the sources of R2 and R3. In the schematic drawing, the other input port is connected to the circuit common.

The non-inverter pin is tied through R1 to circuit common. R1 keeps the input from floating and picking up stray signals. In this amplifier the input changes will all take place on the inverting input circuitry. OK, Here is how this works.

Positive input voltage:

1) The output is sitting at O volts due to the two input voltages being at 0 volts. No voltage drop is across R1, R2 or R3. The Op Amp is at State-2.

2) When a +0.1 V is applied to the inverting signal input terminal, current will start to flow through R2, which will increase the inverting pin voltage above the non-inverting pin. The Op Amp moves to state-1 and the output voltage starts moving down toward the V- voltage. This continues until R3 can pull the voltage applied through the voltage divider to the inverting pin to the same potential as the non-inverting pin.

3) Question: With the input port voltage at +0.1v and both IC inputs back to 0.0V, what is the output voltage? Using Ohms law and the basic R2, R3 voltage divider, we can calculate the output voltage of the amplifier.
There is 0.1 volts across R2 of 10,000 ohms.
R2 current = 0.1/10000 or 0.01mA
There is the same 0.01mA through R3 of 100,000 Ohms.
R3 voltage = 0.00001 X 100000 or 1V.
Output voltage is - 1.0 volts.

SO: It takes 1.0V output in the negative direction to offset the 0.1V input to keep the inverting pin at 0.0V

4) The voltage gain of this amplifier. The GAIN equals R3/R2 or 100K/10K or a GAIN of 10

Negative input voltage

5) Now by decreasing the input port voltage to -0.1V, the inverter pin will signal the op amp to jump to State-3. The output starts increasing. The R3 starts applying that feedback voltage to the inverting pin until that pin returns back to 0.0 volts at which point the op amp switched back to stage-2.

6) Find the new output voltage the same way. There is still a voltage drop across R2 of 0.1 V, just in the other direction this time, so -0.1V.
The divider current is the same as 0.01 mA in the circuit.
Output voltage is +1.0V
The gain is the same at 10

There are a number of different operational amplifiers on the market today. Some models like the CA3033, are no longer available and new IC's show up on the market from time to time. One of the most common over the years is the 741. There are additional letters and numbers as part of the actual part number to designate manufacturers, temperature ranges and alike. Op Amps come in many different pin configurations and specifications. Here is a short list of some common IC's.

Id = general part identification
Slew = slew rate is output voltage change per microsecond.
Volts = power supply voltage range
Tmp = operating temperature limits in degrees C
Cost = general cost range (reference not actual cost)


ID for Single op amp in a package
3140 - 9.0V-Slew, 4Volts to 36Volts -55 to 125 C 2.50 dollars
201 -- 0.5V-Slew, 5Volts to 22Volts -25 to 85 C 0.50 dollars
2904 - 0.1V-Slew, 3Volts to 26Volts -40 to 125 C 1.50 dollars
747 -- 0.5V-Slew, 10Volts to 36Volts 0 to 70 C 0.50 dollars
OP07 - 0.3V-Slew, 3Volts to 18Volts 0 to 70 C 0.50 dollars


ID for Dual and Quad Op Amps
158 - 0.6V-Slew, 3Volts to 30 Volts -55 to 125 C 2.50 dollars
258 - 0.6V-Slew, 3Volts to 30 Volts -25 to 85 C 0.50 dollars
358 - 0.6V-Slew, 3Volts to 30 Volts 0 to 70 C 0.50 dollars
324 - 0.5V-Slew, 4Volts to 32 Volts 0 to 70 C 0.50 dollars

You can see from these sample charts that if you want an IC that is very fast, and can work outside both summer and winter the 3140 is the fastest one here with a wide temperature range, and a higher reference cost.


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