Question

1. Indicate the 4 ways how to polarize a transistor, express drawing.

  2. How is a Mosfet transistor composed?

3. What is one of the main differences between BJT vs. MOSFET transistors?

4. What is BJT Small Signal Amplifiers?

  

5. Talk about Power Amplifiers.

Answer 1

To polarize the BJT transistor, you can use the diode pair model, as showed below. On the left is NPN and on the right is PNP.In the cutoff operation mode, the base-collector (BC) and base-emitter (BE) junctions must be in reverse polarization. The emitter voltage (V_EVE​) must be bigger than the base voltage (V_BVB​). And the later must be smaller than collector voltage (V_CVC​). When the transistor is PNP, the base voltage must be bigger than collector and emitter voltages.

NPN: V_C>V_BVC​>VB​ and V_E>V_BVE​>VB​

PNP: V_B>V_CVB​>VC​ and V_B>V_EVB​>VE​

In saturation mode, the base-collector (BC) and base-emitter (BE) junctions must be in direct polarization. Therefore, the base voltage (V_BVB​) must be bigger than the voltages in collector (V_CVC​) and emitter (V_EVE​). In the PNP, V_BVB​ must be smaller than V_CVC​ and V_EVE​.

NPN: V_B>V_CVB​>VC​ e V_B>V_EVB​>VE​

PNP: V_C>V_BVC​>VB​ e V_E>V_BVE​>VB​

2)A MOSFET is by far the most common transistor in digital circuits, as hundreds of thousands or millions of them may be included in a memory chip or microprocessor. Since they can be made with either p-type or n-type semiconductors, complementary pairs of MOS transistors can be used to make switching circuits with very low power consumption, in the form of CMOS logic.

3)BJTs are preferred for low current applications, while MOSFETs are for high power functions.

4)Small-signal modeling is a common analysis technique in electronics engineering which is used to approximate the behavior of electronic circuits containing nonlinear devices with linear equations. It is applicable to electronic circuits in which the AC signals, the time-varying currents and voltages in the circuit, have a small magnitude compared to the DC bias currents and voltages. A small-signal model is an AC equivalent circuit in which the nonlinear circuit elements are replaced by linear elements whose values are given by the first-order (linear) approximation of their characteristic curve near the bias point.

5)A power amplifier is an electronic amplifier designed to increase the magnitude of power of a given input signal. The power of the input signal is increased to a level high enough to drive loads of output devices like speakers, headphones, RF transmitters etc. Unlike voltage/current amplifiers, a power amplifier is designed to drive loads directly and is used as a final block in an amplifier chain.