Applied Electronics - Outcome 2 - Operational Amplifiers

Operational Amplifiers are components which can be configured to have many uses.  You need to know about 6 of these configurations.

General Information about Op-Amps:

• They have an infinite gain (open loop)
• They have an infinite input impedance (resistance) and so no current flows into the Op-Amp
• They have no output impedance
• There is no difference in the potential between the two inputs

• The Op-Amp has two terminals: A non-inverting terminal (labelled +) and an inverting terminal (labelled -)
• It normally has a positive and a negative supply so all voltages are measured relative to ground.
• Like transistors and MOSFETs if the Op-Amp is fully switched on, it is saturated.

These pictures (from this website) show the pin out diagram of the 741 Op-Amp chip.

Op-Amps can be open loop or closed loop.  If open loop they have no feedback and therefore the gain is infinite.  The first open loop Op-Amp to consider is the comparator.

This compares the two inputs named here as Vi and Vref. Vi goes to the inverting terminal and Vref to the non-inverting terminal. 

• If Vi > Vref then the Op-Amp will saturate negatively so Vo = (85%) -Vcc
• If Vi < Vref then the Op-Amp will saturate positively so Vo = (85%) +Vcc

Due to the characteristics of the Op-Amp in reality, it will saturate at 85% of the supply.

The other open loop configuration you need to know about is the voltage follower.

The two simplest, closed loop Op-Amps are the non-inverting and inverting amplifiers.

Inverting:

The inverting amplifier has a feedback loop with a feed back resistor Rf.  The configuration of this and the input resistor, Ri gives the Op-Amp a gain.  This means that the input voltage is amplified to give the output voltage. 
The gain is found using the equation: Av = -(Rf/Ri)
The output voltage is found by multiplying the Vi by the gain: Vo = Av.Vi, so Vo = -(Rf/Ri) . Vi


Non-Inverting:

The non-inverting amplifier also has a feedback resistor to create a gain.
The gain is found using the equation Av = 1+Rf/Ri
The output voltage is therefore found using the equation Vo = 1+Rf/Ri . Vi

The most important thing to remember about the non-inverting amplifier is to remember about the 1!  This can sometimes make the non-inverting amplifier unsuitable as it always has a gain of more than 1 and you may need a decimal gain.  In this situation you may need to use two inverting amplifiers, one to give the gain and one with a gain of -1 to invert the signal back to a positive one.

Difference Amplifier:

The difference amplifier magnifies the difference between the the two inputs.

Summing Amplifier:

The summing amplifier is used to combine more than one input voltage to give an output voltage.  This is done by multiplying each input by a gain (Rf/Ri) and adding them together.  The inputs can be given different gains depending on their importance/input voltage.  i.e. if a input produces a small voltage, you can make the gain for that input bigger in relation to the others to give it equal importance.

In this way, a summing amplifier can be used as a digital to analogue converter.  Because a microprocessor can only give a high or a low signal, the output will always be either 5v or 0v.  So if you have a signal of 1001, decimal 9, the output voltage should reflect this.  By 'weighting' the input resistors you can create a DAC.