Sunday, August 10, 2008

Operational Amplifiers op-amps

Normal amplifiers (single-ended amplifiers) amplify a single input signal. To get AC output from an amplifier, we use a split (dual) power supply, with 2 DC voltage sources connected in series with the middle point grounded.

Differential amplifiers amplify the voltage difference between input signals. V_out = Av (Input(+) - Input(-) ). Input(-) is inverting input and Input(+) is noninverting input

operational amplifier is a differential amplifier with an extremely high voltage gain (AV = 200,000 or more). Or isa high-gain electronic voltage amplifier with differential inputs and usually a single output. High input impedance at the input terminals and low output impedance are important typical characteristics

Sometimes op-amps are used as signal Comparator. the output of an op-amp will be saturated fully positive if the (+) input is more positive than the (-) input, and ... Comparator can change output voltage states when one input exceeds the other in magnitude. The comparator circuit can also be used as square-wave converter. This technique is sometimes referred to as pulse-width modulation. The problem here is: Any time there is a transition through the reference voltage level, no matter how tiny that transition may be, the output of the comparator will switch states, producing a square wave with "glitches (positive feedback solves this problem).

Negative feedback, Connecting the output of an op-amp to its inverting (-) input is called negative feedback. When the output of an op-amp is directly connected to its inverting (-) input, a voltage follower will be created. Whatever signal voltage is impressed upon the noninverting (+) input will be seen on the output. The circuit will quickly reach a point of stability where the output voltage is just the right amount to maintain the right amount of differential. This stability gives the op-amp the capacity to work in its linear (active) mode, as opposed to merely being saturated fully "on" or "off" as it was when used as a comparator, with no feedback at all. One great advantage to using an op-amp with negative feedback is that the actual voltage gain of the op-amp doesn't matter, so long as it's very large. Negative feedback makes the system self-correcting. The higher the op-amp differential gain, the closer that differential voltage will be to zero.

Positive feedback, the output is bistable: stable in one of two states (saturated positive or saturated negative). Once it has reached one of those saturated states, it will tend to remain in that state, unchanging. an op-amp with positive feedback tends to stay in whatever output state it's already in. It "latches" between one of two states, saturated positive or saturated negative. Technically, this is known as hysteresis. If we add a little positive feedback to the comparator circuit, we will introduce hysteresis into the output. This hysteresis will cause the output to remain in its current state unless the AC input voltage undergoes a major change in magnitude. An oscillator is a device producing an alternating or pulsing output voltage

Divided feedback: by connecting the negative feedback throuth a resistive voltage divider, the output voltage becomes a multiple of the input voltage,  

noninverting amplifier: a negative feedback op-amp circuit with the input signal to the noninverting(+) input. Output voltage becomes the same polarity os the input. adjust the values or R1 and R2 to change the valtage gain. Av = R2/R1 + 1.

Inverting amplifier:  A negative-feedback op-amp circuit with the input signal going to the "bottom" of the resistive voltage divider, with the noninverting (+) input grounded . Its output voltage will be the opposite polarity of the input. Voltage gain is given by the following equation: AV = -R2/R1 .

Instrumentation amplifier is a differential op-amp circuit providing high input impedances with ease of gain adjustment through the variation of a single resistor. Av = 1 + 2R/R_gain

A differentiator circuit produces a constant output voltage for a steadily changing input voltage(C-R). V_out = -RC dV_in/dt

An integrator circuit produces a steadily changing output voltage for a constant input voltage(R-C). V_out = \integ -V_in/RC dt + c, where c = output voltage at start time (t=0)

Follower: It is a noninverting amplifier with gain = 1, sometimes called buffer because of its isolating property (high input impedance, low output impedance)

There must always be feedback at dc in an op-amp circuit. Otherwise the op-amp is guaranteed to go into saturation.

Classic differential amplifier: V_out = R2/R1 (V2 - V1)

Integrators: RC : V_out = 1/RC \int V_in dt + const.
Differentiators: CR : V_out = -RC dV_in/dt

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