# Current Mirrors and Long Tailed Pairs

### Differential Amplifiers – what they do  (Part One)

Long tail pairs or differential amplifiers have one important purpose:  To amplify the difference between two inputs or act as a switch.

In a perfect case the voltage gain of the differential amplifier is the product of the differential gain or Ad and (Vin+  subtract Vin-).

In reality the gain for the two inputs is not equal and there will be an output so the voltage gain is more accurately calculated through the formula below:

The original formula  Ad * (Vin+  –  Vin-) + Ac * ((Vin+ + Vin-)/2)

The Common Mode Rejection Ratio is the ability of the amplifier to cancel voltages that are common to both inputs, a ratio defined by differential-mode gain / common-mode gain.

Single ended output:

Now here is an interesting circuit – the collector load resistors have been removed and it’s place we have two PNP transistors connected in what is known as a current mirror. A really great explanation is given in All About Circuits – Bipolar Junction Transistor – Current Mirrors.

The advantages of using current mirrors:

1. Gain is kept accurate
2. Low input impedance
3. High output impedance

Note:  When the current is sourced the load is connected to PLUS or Vcc (usually 2 x PNP transistors thermally coupled) and in current sinking the load is connected to GND (usually 2 NPN transistors thermally coupled).

See Video on Back to Basics: Transistor Current Sources and Mirrors

Part Two – Practical Examples

Practical examples Differentials and Current Mirrors (Part-II)