Hi:
How would you start a DC analysis of this circuit?
How would you start a DC analysis of this circuit?
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What circuit?How would you start a DC analysis of this circuit?
Hi DennisWhat circuit?
Are you simulating this circuit?LOL.
Yes but I would like to do a manual dc analysis so that I can come up with a set of designAre you simulating this circuit?
The resistor Rf defines the DC bias point for Q1.If it is the circuit shown in post #1, I would start the analysis with out the feedback resistor "Rf"
It will take an iterative process to determine DC bias.I attempted the analysis but went in circles and got no where fast.
This circuit is used by many schools of engineering. Why would they use it if is a poor circuit?Because of negative feedback through Rf and the bias point depending upon the beta gain of the transistors, that is not an easy circuit to analyze, or a very good circuit in practice.
Note that the 10uF output coupling capacitor to the 8 ohm load, means the low frequency cutoff of the amp is about 2kHz.
How do you know that?This circuit is used by many schools of engineering.
Probably because the professors picking the circuits have had little experience designing real circuit.Why would they use it if is a poor circuit?
Hi Dennis:I'd take a simpler approach.
EDIT: corrected arithmetic error for output voltage, was 10.4V.
 Find the base current of Q1 by calculating the current through RB1 and the BE junctions of Q2, Q3, Q1, and RB2. \( I_{RB1} = \frac{V_{Sup}  3*V_{BE}}{RB1+RF+RB2} = \frac{27.9V}{151.22kΩ}=0.18mA \)
 Q1 is now on and beta Q1 (I used 100 for simplicity) times the current calculated into the base of Q1 (18mA) is now flowing in RB1. Giving about 12V at the anode of D1.
 D1, D2, and D3 are now conducting, biasing Q25 on and you can calculate the voltage at the output (10.6V).
Those two values are as close as would be expected between a hand calculation and a simulation.The sim is giving dc Vout = 12.25 V
I said it would take an iterative process to determine DC bias. I only gave 2 iterations.Hi Dennis:
The sim is giving dc Vout = 12.25 V
What does it give if the hfe of Q1 is its minimum value? Or its maximum value?Hi Dennis:
The sim is giving dc Vout = 12.25 V
Here are some data sheets that I have on file.What does it give if the hfe of Q1 is its minimum value? Or its maximum value?
So you now understand why it is such a bad circuit?Dennis / Ian:
If Q1 β = 40 then IC(Q1) = 6.35 mA and if β = 100 then IC(Q1) = 10.25 mA.
RS
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