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:
- Gain is kept accurate
- Low input impedance
- 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