Transistor Operating Points and Biasing Circuits, Slides of Digital & Analog Electronics

Download Transistor Operating Points and Biasing Circuits and more Slides Digital & Analog Electronics in PDF only on Docsity! 15-09-2018 1 CHAPTER 3 TRANSISTORS OPERATING POINT A line joining the saturation and cut off points is called as Load line. Load line drawn over the output characteristic curve, makes contact at a point called as Operating point. Q-point is selected in such a way that irrespective of AC signal swing, the transistor remains in the active region. 15-09-2018 2 FIXED BIAS CIRCUITS FIXED BIAS CIRCUITS Simplest transistor dc bias configuration DC analysis the network can be isolated from the indicated ac levels by replacing the capacitors with an open-circuit equivalent For DC: DC supply VCC can be separated into two supplies for analysis purposes. 15-09-2018 5 VOLTAGE DIVIDER BIASED VOLTAGE DIVIDER BIASED DC components of the voltage divider configurationVoltage-divider bias configuration 15-09-2018 6 BIAS STABILIZATION The stability of a system is a measure of the sensitivity of a network to variations in its parameters. Amplifier employing a transistor the collector current IC is sensitive to each of the following parameters 15-09-2018 7 Variation of Silicon Transistor Parameters with Temperature Shift in dc bias point (Q-point) due to change in temperature 15-09-2018 10 Defining the important parameters of any system Demonstrating the reason for the defined directions and polarities REDRAWN FOR SMALL-SIGNAL AC ANALYSIS 15-09-2018 11 AC EQUIVALENT OF A TRANSISTOR NETWORK IS OBTAINED BY 1. Setting all dc sources to zero and replacing them by a short-circuit equivalent 2. Replacing all capacitors by a short-circuit equivalent 3. Removing all elements bypassed by the short-circuit equivalents introduced by steps 1 & 2 4. Redrawing the network in a more convenient and logical form EMITTER FOLLOWER Output is taken from the emitter terminal of the transistor as shown in Fig. the network is referred to as an emitter- follower 15-09-2018 12 EXAMPLE For the emitter-follower network of Fig, determine a. r e . b. Z i . c. Z o . d. A v . e. Repeat parts (b) through (d) with ro = 25 kohm and compare results. EXAMPLE 5.7 TEXT BOOK CB CONFIGURATION Common-base configuration is characterized as having a relatively low input and a high output impedance and a current gain less than 1