.--|     |----.
                |             |
                |      VCC    |
                |       |     |
          Ri    |  |\   |     |
                |  |  \ |     |
     .-|     |--+--|-   \     |
     |             |      >---+----VO
     |     VI -----|+   /
     |             |  / |
     |             |/   |
     |                  |
     |                 -VCC
0 V -+--------------------------------------o

VO = output voltage
Vi = input voltage
VCC = supply voltage

Another basic amplifier type, made using an operational amplifier (op amp for short). This circuit essentially takes an input voltage, multiplies it by a constant factor (AV or voltage gain) and outputs that voltage. To work out the gain of a non-inverting amplifier, use the following formulae:

      VO         RF
AV = ---- = 1 + ----
      VI         RI

VO = (1 + ---- ) * VI = AV * VI

So, for instance, if you had a signal of two volts and you wanted to multiply this signal by five, you would wire the circuit up so that RF was four times RI. You might choose resistors of 250 KΩ (250 * 103 ohms) and 1 MΩ (1 * 106 ohms) respectively.

AV = 1 + ----- = 1 + 4 = 5

VO = 5 * 2 = 10 V

The voltage at VO would be 10 volts.

A special case of this is the voltage follower, where RF is effectively zero. This results in the equation AV = 1 + 0/RI = 1. Hence, the output voltage is always equal to the input voltage, until saturation occurs (see below).

You can't get more power out of something than you put in, and the same is true for op amps. It is impossible for an op amp to output a higher voltage than its supply voltage (VCC) and in most cases the highest voltage available is around 80% of VCC. Therefore, it is important to make sure that the op amp's power supply inputs have a higher voltage than any expected output voltage. In the above example, a 12 volt supply (+/- 12 V) would be sufficient.

If you graphed VI against VO, the graph should be a straight line through the origin until a certain point, where the required output voltage exceeded the op amp's possible output. When the op amp is in this state it is said to be saturated either positively or negatively. Above this saturation point, VO is no longer proportional to VI.

Incidentally, this can provide some interesting effects using an amplifier with high gain and a wave-form (for instance a sine wave) input. A phenomenon known as clipping can be observed, where the peaks of the output waveform are squared off ('clipped'). This is often known in the area of sound amplification as overdrive or distortion.

Back to op amps

Log in or register to write something here or to contact authors.