Break frequencies for directly coupled transistors

In summary: Still, I'm curious if anyone has any experience or knowledge in this area that they could share...In summary, the problem asks to determine the break frequency at the base of Q2 (which is fine I have no problem with that), but the whole assignment never asks for the break frequency at the COLLECTOR of Q1, then later asks to produce a bode plot of the transfer function. See the image I've linked (pardon the sloppy paint job). However, after extensive research, the student has concluded that adding the Miller effect to the parasitic capacitances (treating them as "parallel") gives nearly the same transfer function results as treating them separately.
  • #1
programmer
13
0

Homework Statement


The problem asks to determine the break frequency at the base of Q2 (which is fine I have no problem with that), but the whole assignment never asks for the break frequency at the COLLECTOR of Q1, then later asks to produce a bode plot of the transfer function. See the image I've linked (pardon the sloppy paint job).

directcoupledtransistorsACmodel.jpg


Homework Equations


f(break) = 1/(2*pi*Ceq*Rth)


The Attempt at a Solution



Well I thought since the capacitor equivalent at collector1 (Cmiller(output) + Cce) wasn't actually in parallel (due to the non-bypassed emitter resistors) with the capacitor equivalent at base2 (Cmiller(input) + Cbe), I would have to treat them seperately and produce two different break frequencies. Is this correct?
 
Physics news on Phys.org
  • #2
still nothing?? Well, I've consulted with my colleague, and he told me that my interpretation of the miller-effect capacitance was incorrect. He told me that they always goes to ground (can anyone verify this??)...and he also told me to ignore the capacitances Cbe and Cce (ignore? never! mwhahaha)...

So looking at this NEW image:

directcoupledtransistorsACmodelpt2.jpg


You can see now that the "break frequency due to the base of Q2" probably means the two miller capacitances added together since now they are truly in parallel. BUT, to be 100% correct, wouldn't I now need a break frequency due to Cmiller(combined), Cce, AND Cbe(3 total)...not to mention the capacitors due to the rest of the circuit.

If I've confused anyone..I'm talking about break frequencies on Bode Plots of the gain vs. frequency response where the slope changes(20dB/decade...40dB/decade..so on), or the -3dB point on the actual transfer function (at least the first break frequency on the midband gain).

Can anyone help me understand this?
 
  • #3
Well after extensive research (PSpice and a graphing calculator), I've concluded that adding the Miller effect to the parasitic capacitances (treating them as "parallel") gives nearly the same transfer function results as treating them separately. I guess it's just another one of those electronic approximations that are far too common.
 

Related to Break frequencies for directly coupled transistors

1. What are break frequencies for directly coupled transistors?

Break frequencies for directly coupled transistors refer to the frequencies at which the performance of a transistor circuit starts to decline. These frequencies are determined by the internal capacitances and inductances of the transistor itself and the external circuit components.

2. Why are break frequencies important?

Break frequencies are important because they determine the maximum frequency at which a transistor circuit can operate effectively. If the operating frequency exceeds the break frequencies, the transistor may start to distort the input signal or become unstable.

3. How do you calculate break frequencies for directly coupled transistors?

The break frequencies for directly coupled transistors can be calculated using the following formula: fβ = 1 / (2πRC), where fβ is the break frequency, R is the resistance in the circuit, and C is the capacitance. This formula applies for both low and high frequency break frequencies.

4. What factors can affect the break frequencies of directly coupled transistors?

The break frequencies of directly coupled transistors can be affected by various factors such as the transistor's internal capacitances and inductances, the external circuit components, temperature, and the operating voltage. These factors can cause the break frequencies to vary from one transistor to another, even if they are of the same type.

5. How can the break frequencies of directly coupled transistors be improved?

The break frequencies of directly coupled transistors can be improved by using high-quality transistors with lower internal capacitances and inductances, and by carefully selecting the external circuit components. Additionally, keeping the operating temperature and voltage within the recommended ranges can also help improve the break frequencies of directly coupled transistors.

Similar threads

Engineering Transistor circuit low-frequency response
    • Engineering and Comp Sci Homework Help
Replies
10
Views
2K
Low and High Frequency Signal Equivalent
    • Engineering and Comp Sci Homework Help
Replies
2
Views
1K
How Do You Calculate Component Values in an FM Transmitter Circuit?
    • Engineering and Comp Sci Homework Help
Replies
5
Views
2K
Transistor bypass capacitor for specific frequency
    • Advanced Physics Homework Help
Replies
1
Views
1K

Hot Threads

Recent Insights

Back
Top