Transistor Differential Amplifier

This Instructable shows how you can make a transistor differential amplifier.

This device is mostly for educational purposes. Neither the transistors or resistors are matched. Thus this circuit is not good at canceling common noise that could be present in both channels.

For more information click on this link:

https://en.wikipedia.org/wiki/Common-mode_rejection_ratio

You can read similar articles on this topic:

https://www.instructables.com/id/Differential-Sensor-Biasing/

https://hackaday.io/page/7005-transistor-modulator

Components: general-purpose transistors - 7, insulated wires, matrix board, 22 kohm resistors - 2, 2.2 kohm resistors - 3, Variable resistor/potentiometer - 10 kohm (minimum value) or 1 Megohm (maximum), power source/batteries (3 V (minimum) or 12 V (maximum)), 1 mm metal wire.

Tools: voltmeter, USB Oscilloscope, pliers.

Optional components: 1 kohm resistor, encasement.

Optional tools: wire stripper, multi-meter, soldering iron, bolts, nuts, washers, drill (electric/manual).

I used PSpice simulation software to reduce the circuit drawing time.

Q1a and Q1b transistors are a current mirror source and have almost matched collector/emitter current.

Calculate the equivalent resistance of Q2a and Q2b transistors when biased at mid point:

Req = (2.2 kohm * 2)||(2.2 kohm * 2) = (4.4 kohm)||(4.4 kohm) = 2.2 kohms

Thus the Rb1 value needs to be at least 2.2 kohm to ensure that Q1b transistor collector-emitter voltage falls to about 0.7 V. This is why I specified the R1b value of 1 kohm. However, waiting for one 1 kohm resistor to arrive in the mail might take many weeks. You can try connecting two 2.2 kohm resistors in parallel that will give you 1.1 kohm resistance.

We are assuming that the transistor pairs have similar characteristics (eg. current gains, temperature drifts or input resistances).

Calculate the Q1b collector current Ic1b:

Because Vbe1a = Vbe1b then: Ic1b = Ic1a and Ib1a = Ib1b

Ic1a + Ib1a + Ib1b = (Vs - Vbe1a) / (R1a + R1b)

Ib1a = Ic1a / Beta

(Beta is the transistor current gain). Typical value is 100.

Ic1a + 2*Ic1a/Beta = (Vs - Vbe1a) / (R1a + R1b)

Ic1a * (1 + 2/Beta) = (Vs - Vbe1a) / (R1a + R1b)

Ic1a = (Vs - Vbe1a) / (R1a + R1b) / (1 + 2/Beta)

= (3 V - 0.7 V) / (1,000 ohms + R1a) / (1 + 2/100)

Maximum Ic1a = 0.00225490196 A = 2.25490196 mA (when R1a = 0 ohms)

Minimum Ic1a = 0.00020499108 A = 204.99108 uA (when R1a = 10,000 ohms)

Determine Ve value:

The Ve value can be determined from specifications graphs if we know the Q1b transistor collector or emitter current.

We can use the Ve value to calculate the output:

Ic2a = Beta * Ib2a = Beta * (VinA - Vbe2a - Ve) / Rb2a

Ic2b = Beta * Ib2b = Beta * (VinB - Vbe2b - Ve) / Rb2b

Vl = Ve + Vc2b = Vs - Ic2b * Rc2b

I did not use a soldering iron.

The white wire is the circuit output.

The orange and yellow wires are the two inputs. The pink wire is connected to the variable resistor. I used a 10 kohm potentiometer. The Red wire is the 3 V power source and the black wire is ground or negative terminal.

The potentiometer is a variable resistor but has three terminals. You can also use a two-terminal variable resistor for this circuit.

I used a cheap plastic box from a local electronics store. I drilled the holes and inserted bolts and nuts.

I drawn the circuit on the box with a permanent marker.

CZ stands for Crazy Circuits:

https://hackaday.io/crazycircuits