In specifications of operational amplifiers there are maximum limits of allowed voltages on pins. Maximum currents are limited as well. So voltage and current they both limit allowed dissipated power Pmax=Umax*Imax.

In well designed circuits Op Amps should have protection circuits from various overloads like short circuit, high common phase voltage level in differential inputs, electrostatic charges, etc.

Earlier operational amplifiers didn’t have built in protection circuits while modern have. Today popular operational amplifiers have internal protection circuits built in and this makes designers life much easier. But protection elements lowers some characteristics of operational amplifiers like operation speed, dynamic range, output signal swing level. Because of this some of operational amplifiers may not have internal protection circuits. In this case you have to take care of it.

Protection from high differential voltage input

When protecting Op Amps from high voltage inputs usually there are protecting diodes connected between input pins.

 

In normal operation (linear mode) when signal input level is up to mV the resistance of diodes is few mega-ohms. Practically diodes have no influence to input signal. But when input voltages opens diodes, then input voltage will be limited to few mili-volts (p-n junction voltage). To limit current flowing through diodes there are resistors needed on each pin.

Short circuit protection

Many op amps have built in protection from short circuit but not all. So where protection is needed there can be resistor connected on op amp output. Resistor has to be connected with feed back – then dynamical output resistance stays low.

Resistor value should be minimal allowing to pass maximal allowed current at voltage applied. For instance if operational amplifier maximal current is 15mA and voltage applied is 5V then resistor value 169Ohm. Resistor connected to the op amp output lowers dynamical range.

Protection from high common phase voltage input

 

In this case if voltage exceed limits then one of diodes opens this way voltage in non inverting input fixes at supply voltage. Input current starts flowing to supply source and if there is voltage regulator as as supply source, then there may be situation, when it may start produce higher voltages on load. To protect circuits from this there should be zener diode connected in supply circuit.

 

Protection from over voltages on output

If circuit connected to op amp output have inductive elements, then on rapid growth of output current there can be over-voltages occur. This way output pin has to be connected via diodes to supply pins:

 

depending on over-voltage polarity one or another diode opens.

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This entry was posted on Wednesday, April 11th, 2007 at 1:09 pm and is filed under Electronics Tutorial. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.