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Wednesday, July 03, 2019

Inverting Amplifier - Op-Amp Designs

This is an Op-Amp Tutorial. Here you see an Operational Amplifier in the Inverting Amp configuration. I have used OP07 as it is almost like an ideal Opamp. The ultra low offset is the best part. Then not really, CA3140 has a Tera Ohm Input resistance but offset not the best. ICL7650 approaches an Ideal Op-Amp and chopper stabilized.

Inverting Amplifier - Op-Amp Designs

These parameters are good for DC Amps. Sensors, Strain Gauge Bridges Signal Conditioning need them. Then when you want a better AC performance, you can use a LF353 with a nice slew rate. Here the DC merits are not of much use.

Inverting Amplifier - Op-Amp Circuits

Input Impedance of this module is Ri as pin 2 is at virtual ground, the opamp with feedback tries to maintain pin 2 and 3 at same potential pin 3 is at 0V hence pin 2 is at virtual ground. Clamping diodes protect OpAmp, Rf + Ri is between 5kE and 1ME as an opamp may be able to drive around say 5mA max

Current into node pin 2 = Vin/Ri if Vin is +ve it raises potential at pin 2, in order to bring it to 0V the OpAmp sucks away the current by turning its output negative the current leaving pin 2 node is also Vin/Ri. Then Vout is given by Vin/Ri * Rf as per V=IR ohms law. Most OpAmps output swings around 1v less than VCC/VDD for full swing use CA3130 this is a FET input OpAmp, and has low bias currents in pico amps.

Slide the Potentiometers just like you would operate a Sliding Control. Drag the Knob on Pot to increase or decrease the resistance. The Resistance is shown is blue letters and dynamically alters value as you slide the pot. Both Rf and Ri have Pots which are variable resistors.

The mV Source is varied by just moving mouse pointer over the two buttons, no clicking. This reduces finger strain and also you have a long lasting mouse. The mV buttons are special, the variation picks up speed if you let the mouse pointer remain on the button. This is Ramp-up and Ramp-down. This enables you to set it faster with just two buttons. This mV Source sets Vin.

The mV DPM at Output shows Vout. Vin and Vout are in mV.  The formula you know

Vout = Vin * (-1) * (Rf / Ri)

A real pot has a Minimum 0 ohms value, but in these pots i have shown 10K as min. Even in a real design, depending on input source impedance and output voltage the resistors are better kept high. Never go less than 4.7K, the OpAmp loads or the source, even another opamp stage or sensor loads. These can be resolved by using buffers or power amps respectively, if such a need arises. For all analog computing/signal conditioning use 10K min. Battery operated designs have 100K as min and FET amps only.

Values are to give an idea, keep in mind Battery Consumption, Source Impedance and what the Op-Amp need to drive. Most Op-Amps today are short circuit protected but even little loading will upset your equations. Very high resistance, even above 1M, careful PCB design is required. Leakage currents on PCB will play spoilsport. A little sunshine and moisture, there can be copper sulfate formation on terminals and tracks on exposed PCB. There is no formula for that. Theory and Practice in Tandem, this works.

Never drive any Opamp pin to a voltage higher than the supply +/- dual supply. Use clamping diodes, read specs well.

The Force does not happen to everybody. You have to master your Theory and Formulas. You then have to Experiment, Try out and Understand in detail. Then you shall modify, create & Juggle circuits !

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