Transistors

Many common circuit elements are modeled as dependent sources—that is, the circuit element provides power (voltage and/or current) to the circuit, but the voltage or current provided to the circuit depends upon a voltage or current somewhere else in the circuit.

Dependent sources can be either voltage or current sources; Fig. 1(a) shows the symbol for a dependent voltage source and Fig. 1(b) shows the symbol for a dependent current source. Since each type of source can be controlled by either a voltage or current, there are four types of dependent current sources:

  1. Voltage-Controlled Voltage Source (VCVS). Figure 2.
  2. Current-Controlled Voltage Source (CCVS). Figure 3.
  3. Voltage-Controlled Current Source (VCCS). Figure 4.
  4. Current-Controlled Current Source (CCCS). Figure 5.

Figure 1. Dependent voltage source (V) and dependent current source (i).

Figure 2. Voltage controlled voltage source.

Figure 3. Voltage controlled current source.

Figure 4. Current controlled voltage source.

Figure 5. Current controlled current source.

A number of useful circuit elements behave like dependent sources. The primary ones include Metal Oxide Semiconductor Field Effect Transistors (MOSFETs), Bipolar Junction Transistors (BJTs) and operational amplifier (op-amp) based circuits. Operational amplifiers are introduced in later exercises.

MOSFETs

MOSFET stands for Metal-Oxide-Semiconductor Field-Effect Transistor. There are two basic types of MOSFETs: n-channel and p-channel; the discussion presented here is for n-channel MOSFETs, though similar concepts apply to p-channel MOSFETs. A MOSFET is a three-terminal device; the symbol commonly used to represent a MOSFET in circuit diagrams is shown in Fig. 6. The three terminals of the device are called the source (S), the drain (D) and the gate (G). Your parts kit includes a ZVN2210A MOSFET. The physical appearance of this MOSFET is shown in Fig. 7, along with the relative locations of the drain, gate, and source for that model MOSFET.

Figure 6. MOSFET symbol.

Figure 7. ZVN2110A MOSFET.

An extremely simplified discussion of a MOSFET's operation is as follows: A “channel” is opened in the MOSFET by application of a voltage at the gate of the MOSFET. This channel allows current to flow from the drain to the source of the MOSFET (iD in Fig. 6). Thus, if a power supply is connected to the drain of the MOSFET, the MOSFET can be used to control the power supply's current. Increasing the gate voltage increases the current out of the power supply. A rough analogy to this process is a valve placed at the base of a water tank—opening the valve allows water to flow out of the tank. Likewise, increasing the gate voltage allows current to “flow” out of the power supply. A MOSFET, therefore, in conjunction with a power supply, can act as a voltage controlled current source in which the drain current is controlled by the gate voltage. One important aspect of MOSFET operation is that little or no current is required at the gate of the MOSFET. Thus, essentially no power must be delivered by the voltage applied to the gate.