I need help with the following question:
Where to connect capacitor Cp and how to find fall and rise time of Vout?
What is the value of Vout in this case?
Hi User,
I have chosen to answer the easy part- the rest of the circuit functions make my head hurt due to the arcane design and the bad wording of the questions.
The capacitor can be connected across the 5K resistor or it can be connected from the collector of the second transitor to 0V; it makes no difference, but you would have thought that this would have been made clear by the exam question.
The output wave form can then be derived as follows:
(1) when the second transistor turns off: from the exponential function for a capacitor and the 5k resistor in parallel.
(2) when the second transitor turns on: from the formula for a capacitor and 5K resistor in parallel being fed by a constant current, the constant current being Ib * beta of the second transistor.
From this you can see that the waveform at the second transistor collector will be asymmetrical with different rising and falling edges. This is a common characteristic with circuits that rely on a resistor to supply current in one of the output directions. So, in your circuit, you will relatively fast falling edges but slow rising edges.
It would be a big help if you could name each component: R1, R2, Q1 and Q2 etc.
It is worrying that such a bad design, as you show, should be an exam question.
You could achieve the same NAND function with one transistor rather than two. Why there is negative feedback on the base of the first transistor is a mystery- it shows confused thinking.
spec
PS: By the way, 0.6V is a perfectly reasonable assumption for the forward drop of a small signal transistor VBE and a small signal diode, assuming both are operating with a low forward current. Some people assume 0.7V, others assume 0.65V and others assume 0.6V. In practice, transistors and diodes of the same type and same circuit conditions, will have varying forward voltages anyway.