Thursday, March 13, 2014

Two Op-Amp Differential Amplifier

The Input Impedance of this module is very high and is symmetric. This circuit can be used for strain gauges and for four wire measurements. If inputs are in mV use OP07. The merit is that it uses only 2 OpAmps yet has high differential Input Impedance.

Dual Differential Amp - Interactive Simulation

Above page has the Interactive Tutorials using Virtual Electronic Components of this example.

Differential Amplifier using only Two Op-Amps, Interactive Tutorial Gadget. Instrumentation Amplifier. This is used in the front end of Signal Conditioners for Load, Strain and Pressure Measurement. High Z and Gain, Easy to configure Zero and Span.

Two Op-Amp Differential Amplifier

Vout = (Vp - Vn) * (Rf+Ri)/Ri

The Outputs of Opamps are low impedance but still have limits they cannot drive more than a few mA of Current into the Load. If low ohmic value loads are to be applied use external transistors as amplifiers. If inputs Vn-Vp are floating Outputs may be random or Oscillating, it is good to have a bias network of 10M resistors to a potential even zero or COM this enables Vout when input floats.

Wednesday, March 12, 2014

Non-Inverting Amplifier - Op-Amp Circuits

The input impedance of this module is very high, if U1 is OP07 it is in mega ohms, use CA3140 or LF356 fet input opamps to get 1 tera ohm input impedance, but for high gains OP07 is better as it is ultra low offset, this is a good amplifier for sensor outputs, as in a DC Circuit.

Non-Inverting Opamp Interactive Simulation


Non-Inverting Amplifier - Op-Amp Circuits

Vout = Vin * (Rf + Ri) / Ri

The zener diodes protect the opamp inputs, R1 limits current during high voltage inputs and R1 and C1 form a filter to remove ac components C1 should be a plastic type as ceramic and electrolytic caps are leaky. A large C1 will slow the response time, the sum of Ri + Rf should be greater than 5k so that output is not loaded. also do not connect output to voltages more than vcc/vdd it will blow Opamp.

Monday, March 10, 2014

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.

Inverting Opamp - Interactive Simulation

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.
Inverting Amplifier - Op-Amp Circuits

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

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