High voltage input question?

Input and Output Voltage question in Transistor?

• is the relationship of the phase bet. Vi and Vo in PNP the same as in NPN? Why?

• Answer:

  depends on the circuit! To make a circuit with amplification, we can connect the transistor in a few different ways: we can ground the emmitter, connect the input to the base and take the output from the collector. This is called common emmitter. We usually create an output voltage by passsing the collector current through a resistor. In this case, we have a 180 degree phase shift from output to input. Sometimes we use a large value inductor (choke) instead of a resistor. Then we will have a different phase relationship (90 degrees ideally). This happens because the common emmitter circuit works through 'transconductance', so a change in input voltage give a proportional change in the output collector current. So if we increase the input voltage, the output current increases, which increases the voltage across a resistive load, which decreases the output voltage at the collector. So the input signal is inverted at the output for a resistive load. It doesnt matter if its NPN or PNP, becuase the polarity of all signals simply swaps when going between the two. We can also connect the transistor in 'common base', where the base is grounded, and the input is at the emmitter, the ouptut is at the collector. This works similarly to the common emmitter, except we have effectively changed the polarity of the input (it is from emmitter to base now, rather than from base to emmitter), so we dont see the 180 degree phase shift like with the common emmitter (for a resistive load!). It still doesnt matter for NPN or PNP, only the polarity of the bias currents and voltages swap.