EENG 3520: Electronics II Lecture 3 Oluwayomi Adamo

MOS Field-Effect Transistors Physical Structure and Physical Operation Figure 4.1 Physical structure of the enhancement-type NMOS transistor: (a) perspective view; (b) cross-section. Typically L = 0.1 to 3  m, W = 0.2 to 100  m, and the thickness of the oxide layer (t ox ) is in the range of 2 to 50 nm.

MOS Field-Effect Transistors Physical Structure and Physical Operation (cont.) Tri-Gate Transistor

MOS Field-Effect Transistors Physical Structure and Physical Operation (cont.)

Bipolar Junction Transistor Physical Structure and Physical Operation

Bipolar Junction Transistor Physical Structure and Physical Operation (cont.) ModeEBJCBJ CutoffReverse ActiveForwardReverse Reverse ActiveReverseForward SaturationForward BJT Modes of Operation > 0.5 V

Bipolar Junction Transistor Physical Structure and Physical Operation (cont.)

Bipolar Junction Transistor Graphical Representation of Transistor Characteristic Figure 5.16 The i C – v BE characteristic for an npn transistor.Figure 5.18 The i C – v CB characteristics of an npn transistor.

Bipolar Junction Transistor The BJT as an Amplifier and as a Switch Figure 5.26 (a) Basic common-emitter amplifier circuit. (b) Transfer characteristic of the circuit in (a). The amplifier is biased at a point Q, and a small voltage signal v i is superimposed on the dc bias voltage V BE. The resulting output signal v o appears superimposed on the dc collector voltage V CE. The amplitude of v o is larger than that of v i by the voltage gain A v. Grounded-emitter, Common-emitter (CE) Voltage-controlled current source

Bipolar Junction Transistor Assumption: Only DC voltage are applied |V BE | = 0.7 V (Active mode) |V BE | = 0.7 V, |V CE | = 0.2 V (Saturation mode) Analysis Method: In which mode is the transistor operating? Assume one mode Determine voltage and current Check for consistency. Assume active mode, check v CB : > -0.4V (npn) < 0.4V (pnp) Assume saturation, I C /I B < , or  forced <  BJT Circuits at DC

Bipolar Junction Transistor Example : To determine the voltages at all nodes and the currents through all branches. Assume that the transistor  is specified to be at least Assume active: V BE = 0.7V, V E = 6 – 0.7 = 5.3 V 2. I E = 5.3 / 3.3 = 1.6mA 3. I C =  I E = (50/51)I E  1.6 mA 4. V C = 10 – 1.6 x 4.7 = 2.48 V < V B 1. Assume saturation: V BE = 0.7V, V E = 6 – 0.7 = 5.3 V 2. I E = 5.3 / 3.3 = 1.6mA 3. V CE = 0.2 V, V C = V E + V CE = = 5.5 V 4. I C = (10 – 5.5) / 4.7 = 0.96 mA 5. I B = I E – I C = 1.6 – 0.96 = 0.64 mA 6.  forced = I C / I B = 0.96 / 0.64 = 1.5 < 50

Bipolar Junction Transistor Biasing in BJT Amplifier Circuits Goals: To establish a constant dc current I C in the collector of the BJT Insensitive to variations in temperature and to the large variations in the value of  To allow for maximum output signal swing Two obvious Examples: Small V BE, Large I C I C depends on 

Bipolar Junction Transistor Biasing in BJT Amplifier Circuits (cont.)

Bipolar Junction Transistor Biasing in BJT Amplifier Circuits (cont.) Swing range is determined by V CB

Bipolar Junction Transistor Figure 5.47 (a) A BJT biased using a constant-current source I. (b) Circuit for implementing the current source I. SAME V BE CURRENT MIRROR Biasing in BJT Amplifier Circuits (cont.)

Bipolar Junction Transistor Small Signal Models

Bipolar Junction Transistor Small Signal Models (cont.)

Bipolar Junction Transistor Application of the Small-Signal Equivalent Circuits Systematic process for the analysis of BJT 1.Determine the dc operating point of the BJT and in particular the dc collector current I C 2.Calculate the values of the small-signal model parameters: g m, r , r e 3.Eliminate the dc sources by replacing each dc voltage source with a short circuit and each dc current source with an open circuit 4.Replace the BJT with one of its small-signal equivalent circuit models. 5.Analyze the resulting circuit to determine the required quantities (voltage gain, input resistance)

Bipolar Junction Transistor Application of the Small-Signal Equivalent Circuits Known Unknown

Homework 6.59, 6.95, 5.9, 5.26, 5.76