Differential Mode Gain: Positive Gain With -Vid/2?

Click For Summary
SUMMARY

The discussion centers on the differential mode gain of a specific differential amplifier circuit, where the output behavior contradicts the expected results based on input polarity. When using the second differential input (-Vid/2), the calculated gain is positive, while the first input (Vid/2) yields a negative gain. This is contrary to the anticipated behavior where negative inputs should produce negative gains and vice versa. The circuit analysis involves small signal models and calculations using parameters such as ib, rpi, Ree, Rc, and Rcc, leading to confusion regarding the gain calculations.

PREREQUISITES
  • Understanding of differential amplifiers and their operation
  • Familiarity with SPICE simulation tools for circuit analysis
  • Knowledge of small signal models in electronics
  • Basic concepts of transistor parameters such as beta and input/output resistance
NEXT STEPS
  • Study the principles of differential amplifier gain calculations
  • Learn how to perform SPICE simulations for differential circuits
  • Explore small signal analysis techniques in electronic circuits
  • Investigate the effects of transistor parameters on amplifier performance
USEFUL FOR

Electronics engineers, students studying amplifier design, and anyone involved in circuit simulation and analysis will benefit from this discussion.

FOIWATER
Gold Member
Messages
434
Reaction score
12
In the following diff amp, only one output is taken from collector two.

When I calculate the differential mode gain using the second differential input (-Vid/2) I get a positive gain. When I calculate the differential mode gain using the first differential input (Vid/2) I get a negative gain.

A spice simulation of this circuit shows that the output is positive.

I would then expect to get a negative gain for (-vid/2) and a positve gain for (vid/2)

Does someone know why that is not the case?
 

Attachments

  • pic.png
    pic.png
    5.9 KB · Views: 511
Engineering news on Phys.org
Not sure how you are calculating gain. The basic operation is that when V+ is greater than V-, Q2 will try to turn off.
 
I made a small signal circuit out of the above using a differential mode half circuit. with the input as -Vid/2, I have -Vid/2 = ib(rpi + (1+b)*Ree/2) <-- (virtual ground between Ree/2 for diff mode.)

Vodm = -beta*ib*(Rc||Rcc/2) <--- another virtual ground between Rcc/2

Solving for adm = vodm/vid I get a positive number.

However, I would expect my negative input to be multiplied by a negative gain and give a positive output.

The same is true for Vid/2, I would expect it to be multiplied by a positive gain.

Am I missing things?

Thanks for the feedback
 

Similar threads

Types of circuits that have a nonlinear response to R
  • · Replies 29 ·
Replies
29
Views
2K
Gain calculation of tube class AB output stage
  • · Replies 10 ·
Replies
10
Views
3K
DC input random offset voltage for diff. amplifier with current-mirror load
  • · Replies 2 ·
Replies
2
Views
2K
Op-amp compensation of cascade fudges audio signal oddly
  • · Replies 3 ·
Replies
3
Views
2K
Gain Magnitudes Equal: Rejecting Common Mode Signals
  • · Replies 6 ·
Replies
6
Views
2K
Common mode differential mode signals
  • · Replies 2 ·
Replies
2
Views
6K
Engineering Diff gain of a push pull degenerated differential pair
  • · Replies 6 ·
Replies
6
Views
3K
HX711 query (Amp/ADC for load cells)
  • · Replies 4 ·
Replies
4
Views
2K
Differential Amplifier Common Mode Thevenin Equivalent
  • · Replies 1 ·
Replies
1
Views
2K
What is the "observer" in PID control?
  • · Replies 12 ·
Replies
12
Views
4K