1 Single-Stage BJT Amplifiers and BJT High-Frequency Model Asst. Prof. MONTREE SIRIPRUCHYANUN, D. Eng. Dept. of Teacher Training in Electrical Engineering, Faculty of Technical Education King Mongkut s Institute of Technology North Bangkok http://www.te.kmitnb.ac.th/msn mts@kmitnb.ac.th 1 Single-Stage BJT Amplifier Configurations There are three basic configurations for single-stage BJT amplifiers: Common-Emitter (Common-Source) Common-Base (Common-Gate) Common-Collector (Common-Drain) Let s look at these amplifier configurations and their small-signal operation 224510 Advanced communication circuit design 2
2 Common-Emitter Amplifier First, assume R e = 0 (this is not r e, but an explicit resistor) The BJT is biased with a current source (with high output impedance) and a capacitor connects the emitter to ground. Cap provides an AC short at the emitter for small time-varying signals but is an open circuit for DC signals Can redraw the circuit with an equivalent circuit that replaces the BJT with its hybrid-π model R s B g m C v s r π r ο R C v o E 224510 Advanced communication circuit design 3 CE Amp with Emitter Degeneration Now, assume R e 0. First, find R i voltage applied to the base is across r e and R e base current is and let s find R i this tells us adding R e increases the input resistance Can design the desired R i by setting R e 224510 Advanced communication circuit design 4
3 To determine the voltage gain, first find the gain from the base to the collector (ignore r o b/c it complicates the analysis considerably) NOTE: Voltage gain between base and collector is equal to ratio of total resistance in the collector to the total resistance in the emitter. To find the total gain, Characteristics with R e : gain is lower, but also less dependent on β input resistance is higher allows higher input signal voltage 224510 Advanced communication circuit design 5 Common-Base Amplifier Ground the base and drive the input signal into the emitter through a coupling capacitor (only passes ac signals) Model the small signal approximation with a T-model current source is an AC open and C C is an AC short 224510 Advanced communication circuit design 6
4 First, we can see that To find the gain, solve for v o The output impedance is just CB amp characteristics: voltage gain has little dependence on β gain depends critically on R s is non-inverting most commonly used as a unity-gain current amplifier or current buffer and not as a voltage amplifier: accepts an input signal current with low input resistance and delivers a nearly equal current with high output impedance most significant advantage is its excellent frequency response (which we will see later) 224510 Advanced communication circuit design 7 Common-Collector Amplifier (Emitter Follower) The last basic configuration is to tie the collector to a fixed voltage, drive an input signal into the base and observe the output at the emitter Also called an emitter follower since the emitter follows the input signal Used for connecting a source with a large R s to a load with low resistance 224510 Advanced communication circuit design 8
5 Redraw the circuit to have r o in parallel with R L now, find R i when r e << R L << r o notice the amplifier has large input resistance Find the gain with two voltage dividers gain is less than unity, but close (to unity) since β is large and r e is small 224510 Advanced communication circuit design 9 High-Frequency BJT Model B r b C μ C C π r π g m r ο E In BJTs, the PN junctions (EBJ and CBJ) also have capacitances associated with them C μ is the reverse-biased CB junction Where m jc is between 0.2 and 0.5, V 0 is between 0.5V and 1V C π represents the capacitance of the forward-biased EBJ which exhibits both the junction cap and diffusion cap Where C je is the junction cap and τ F is the base-transit time At high frequencies, the base resistance can also an important role in device operation 224510 Advanced communication circuit design 10