Definition of Bipolar Junction Transistor (BJT)

The Bipolar Junction Transistor (BJT) is an active device. In simple terms, it is a current controlled valve. The base current (IB) controls the collector current (Ic).

Regions of BJT Operation

Cut-off region: -

The transistor is off. There is no conduction between the collector and the emitter.(IB=0 therefore Ic=0)

Active region: -

The transistor is on. The collector current is proportional to and controlled by the base current (Ic=ßIB) and relatively insensitive to VCE. In this region the transistor can be an amplifier.

Saturation region: -

The transistor is on. The collector current varies very little with a change in the fill fully base current in the saturation region. The VCE is small, a few tenths of volt. The collector current is strongly dependent on VCE unlike in the active region. It is desirable to operate transistor switches in or near the saturation region when in their on state.

Rules for Bipolar Junction Transistors (BJTS)

1.For an npn transistor, the voltage at the collector VC must be greater than the voltage at the emitter VE by at least a few tenths of a volt; otherwise, current will not flow through the collector-emitter junction, no matter what the applied voltage at the base. For pnp transistors, the emitter voltage must be greater than the collector voltage by a similar amount.

2. For the npn transistor, there is a voltage drop from the base to the emitter of 0.6 V. For a pnp transistor, there is also a 0.6 V rise from the base to the emitter. In terms of operation, this means that the base voltage VB of an npn transistor must be at least 0.6 V greater that the emitter voltage VE; otherwise, the transistor will not pass emitter-to-collector current. For a pnp transistor, VB must be at least 0.6 V less than VE; otherwise, it will not pass collector-to-emitter current.

Basic Equations For The BJT

For npn: VB > VE + 0.6 V

For pnp: VBE – 0.6 V

For both npn and pnp anytime

IE = IC + IB

For both npn and pnp only in the active region

IC = h FE IB = ß IB

IE = IC+ IB = (ß+1) IB ~ ß IB

Common Circuit Application(Voltage-Controlled Resistor)

V_GS must be between zero and V_GS.off .

Common Circuit Application(Source Follower)

The simple source follower is shown below. The improved version is shown at the right. The lower JFET forms a current source. The result is that V_GS is held constant, removing the defects of the simple circuit.

Common Circuit Application(Current source)

The drain current is set by R_S such that V_GS = I_DR_S. Any value of current can be chosen between zero and I_DSS.

Common Circuit Application(Voltage Controlled Switch)

For the on state the gate voltage V_GS = 0 and for the off state |V_GS| > |V_GS.off| (of greater magnitude then V_GS.off and with the same sign). The sign of the voltage depends on the type of FET, negative for n-channel and positive for p-channel.

Regions of JFET operation

Cut-off Region: -

The transistor is off. There is no conduction between the drain and the source when the gate-source voltage is greater than the cut-off voltage (I_D = 0 for V_GS > V_GSoff)

Active Region (also called the Saturation Region): -

The transistor is on. The drain current is controlled by the gate-source voltage (V_GS) and relatively insensitive to V_DS. In this region the transistor can be an amplifier.

In the active Region I_D = I_DSS (1 – V_GS / V_GSoff) ²

Ohmic Region: -

The transistor is on, but behaves as a voltage proportional to the source-drain voltage and is controlled by the gate voltage.

I_D = I_DSS [ 2 (1 – V_GS / V_GSoff) V_DS / - V_GSoff - (V_DS / V_GSoff) ² ]

In the Ohmic Region: R_DS ≈ V_GSoff / 2I_DSS (V_GS - V_GSoff) = 1 / g_m

Definition of Field effect Transistor (FET) and FET Schematic Symbols

The Field effect Transistor (FET) is an active device. In simple terms, it is a voltage controlled valve. The gate-source voltage (VGS) controls the drain current (ID).

The FET is a three terminal device like the BJT, but operates by a different principle. The three terminals are called the source, drain and gate. The voltage applied to the gate controls the current flowing in the source-drain channel. Because no current flows through the gate, the input impedance of the FET is extremely large (in the range of 10¹⁰ - 10¹⁵ Ω). The large input impedance of the FET makes them an excellent choice for amplifier inputs.

The two common families of FETs, the junction FET (JFET) and the metal oxide semiconductor FET (MOSFET) differ in the way the gate contact is made on the source-drain channel.

FET Schematic Symbols: -

Two Versions of the symbols are in common use. The symbols in the top row depict the source and drain as being symmetric. This is not generally true. Slight asymmetries are built into the channel during manufacturing which optimize the performance of the FET. Thus it is necessary to distinguish the source from the drain. In this class we will use the asymmetric symbols found on the bottom row, which depict the gate nearly opposite the source. The designation n-channel means that the channel is n doped and the gate is p doped. The p-channel is complement of n-channel.