Differential signaling using a pair of wires is highly useful for sending signals between circuits. It makes it possible to remove several types of noise and interference added to the signal along its path from transmitter to receiver. USB and wired Ethernet connections are common examples of differential signaling in practice.
The desired signal is applied as the difference between the two wires and, ideally, the noise voltages/currents are added equally and inphase to each conductor of the pair. The receiver simply takes the difference of the pair as the signal quantity, therefore removing all of the noise.
There are 3 nodes that participate in a differential pair of signals: a common reference defined at the receiver’s end of the circuit, and the two signal nodes A and B (or [+in and in], or [inp and inm]) The node voltages Va and Vb (measured with respect to the circuit’s reference node) completely describe the values.
However, it is much more useful to define another set of quantities:
Given the set of differential and commonmode voltages for a pair of nodes, it is a small matter of algebra to know the individual node voltages:
See Two ways of specifying the voltage at a pair of nodes. for a graphical version of these conversion equations.
 Some additional points to remember


In both systems, two terms are required to completely specify the potentials at the two nodes.

The equations use terms in lower_{UPPER} notation, the total or physical quantity, to emphasize that these two systems are always valid.

… meaning they are also valid for DC quanitities (UPPER_{UPPER} notation) and for smallsignal quantities (lower_{lower} notation).

1. Lab goals
Reminder: we are using the AD2 specifically because they are able to output two phaselocked waveforms and the input channels are differential. There is no need to use "Math" to subtract channel 2 from channel 1 → just directly attach the two wires of each channel to the two nodes that are to be subtracted.
2. Procedure
2.1. DC conditions
Make these measurements to no more than 3 digits of precision. Only in section 2.4 should you need to use the Keysight multimeters.

Build the circuit of Differentialpair schematic.

Apply 0 V to both inputs ina and inb.
Measure the node voltages and currents through the 3 resistors:
Name 
Value 
V_{outA} 

V_{outB} 

V_{E} 

I_{R1} 

I_{R2} 

I_{RE} 
3. Amplifier measurements

Configure the AD2’s function generators to output a pair of signals which yield a pure commonmode signal of:

Be sure to configure the Wavegen for the AD2’s Waveforms software from "No synchronization" to "Auto synchronization". This ensures that both signal generators start at exactly the same time. Weird waveforms will be the result if your forget this!

Measure the three commonmode voltage gains in this circuit.

These are the signal amplitudes, in volts peaktopeak or volts RMS (preferred),
→ do NOT use "Math" to divide one oscilloscope channel by the other. 
You can predict the first two singleended gains by viewing the circuit two separate commonemitter amplifiers with emitter resistor values of 2R_{E} (≈ − R_{c} / 2R_{E}).

Predict the common to differential mode gains by thinking about the circuit’s symmetry.


Measuring the differential output \(v_{out,\,d}\) is where you can use the fact that the AD2’s input channels are differential. There is no need, like there is with a benchtop oscilloscope, to use two probes, one for each node A and B, and then subtract the channels using the Math function. Simply connect the + and − leads of the AD2 channel directly across nodes outA and outB.

Configure the AD2’s function generators to output a pure differential triangular signal of:

Plot the largesignal differentialinput to differentialoutput transfer function using an XY display, \((V_{outA}  V_{outB}) \text{ versus } V_{in,\,d}\).

From this XY plot, determine the maximum differential input amplitude that still gives an undistorted differential output signal. (hint: it will be within a small factor of the thermal voltage \(V_T\))

Reduce the differential input amplitude to this value and measure the following gains. They will be about 90 or 180 V/V.

Build the circuit of Differentialpair with tail current source schematic.

Determine the closest E12 value for R3 to give the same current as was through R_{E}. Verify this current by measurement. Hint: what is the voltage across R3?

Measure the same six gains on this circuit as on Differentialpair schematic (three \(A_{cm}\) variations and three \(A_d\) variations).

Measure input currents and the baseemitter voltages. Place a 10k resistor in series with the bases of each of Q1 and Q2. Set \(v_{i,\,cm} = 0\) and \(v_{i,\,d} = 0\) using the function generators. Measure the base currents of Q1 and Q2 by measuring the DC voltage drop across the series resistors. This is a great opportunity to use the nice, new, Keysight meters!
Name 
Value 
I_{B1} 

I_{B2} 

V_{BE1} 

V_{BE2} 
4. References

[341notes] D. White, ECE 341 Class notes 2018 folder, https://drive.google.com/folderview?id=1hUN1Xicpr9tpCsL2937jfNaCxgpyLT3L

[341docs] D. White, ECE 341 reference documents folder, https://drive.google.com/folderview?id=0B5O5cSaA0tEQYVpaSnJxMGFrdHM

[AoE] P. Horowitz and W. Hill, The Art of Electronics, 3rd ed. Cambridge University Press, 2015. https://artofelectronics.net

[LAoE] T. Hayes, Learning the Art of Electronics: A HandsOn Lab Course, Cambridge University Press, 2016. https://learningtheartofelectronics.com

[LEC] Tony R. Kuphaldt, Lessons in Electric Circuits, Source version: https://www.ibiblio.org/kuphaldt/electricCircuits/, All About Circuits version: https://www.allaboutcircuits.com/textbook/

[CLbook] Michael F. Robbins, CircuitLab, Ultimate Electronics: Practical Circuit Design and Analysis, https://www.circuitlab.com/textbook/

[TCA] Alfred D. Gronner, Transistor Circuit Analysis, Simon & Schuster, 1970, https://archive.org/details/TransistorCircuitAnalysis

[] Neil Weste and David Harris, CMOS VLSI Design  A Circuit and Systems Perspective, 4th edition. AddisonWesley, 2011. http://pages.hmc.edu/harris/cmosvlsi/4e/index.html

[Guidebook] D. White, Guidebook for Electronics II. https://agnd.net/valpo/341/guidebook

[] H.K. Gummel, H.C. Poon, An Integral Charge Control Model of Bipolar Transistors. Bell System Technical Journal, 49: 5. MayJune 1970 pp 827852. https://archive.org/details/bstj495827

[ROHM] ROHM Semiconductor, Electronics Basics, http://www.rohm.com/web/global/en_index

[vishayeseries] Vishay, Standard Series Values in a Decade for Resistances and Capacitances, https://www.vishay.com/docs/28372/eseries.pdf