emitter follower

The emitter follower (also known as the emitter output device, referred to as the emitter follower or follower) is a circuit with a common-collector connection, as shown in the figure below. It inputs a signal from the base and outputs a signal from the emitter. It has the characteristics of high input impedance, low output impedance, and the input signal and the output signal have the same phase. 1. The main indicators of the emitter follower and its calculation 1. The input impedance is from the circuit (b) in the above figure, from terminals 1 and 1` to The input impedance seen on the right is: Ri=Ui/Ib=rbe+(1+β)ReL. In the formula: ReL=Re//RL, rbe is the input resistance of the transistor. For low-frequency and low-power tubes, its value is: rbe=300+ (1+β)(26 millivolts)/(Ie millivolts) In the circuit (b) above, if you look to the right from terminals b and b', the input impedance is Ri=Ui/Ii=Rb//Rio. It can be seen from the above formula that the input impedance of the emitter follower is (1+β) times higher than the input impedance rbe of the general common emitter circuit. 2. The output impedance is Es=0. From e and e" in the figure above, the output impedance is: Ro=Uo/Ui=(rbe+Rsb)/(1+β), where Rs= Rs//Rb, if the output impedance looking to the left from the output terminals 0 and 0' is Ro=Ro//Reo 3. The voltage amplification factor is obtained according to the equivalent circuit in (b) above: Kv=Uo/Ui=( 1+β)Rel/[Rbe+(1+β)Rel], where: Rel=Re//RL, when (1+β)Rel>>rbe, Kv=1, usually Kv<1. 4. Current The amplification factor is calculated based on the equivalent circuit in (b) above: KI=Io/Ii=(1+β)RsbRe/(Rsb+Ri)(Re+RL) where: Rsb=Rs//Rb,Ri=rbc+ (1+β)Relo Usually, the emitter follower has the function of current and power amplification. 2. Practical circuit of the emitter follower. The figure below is a circuit used by a high-frequency amplifier. The signal is output by a coaxial cable. The characteristic impedance of the cable. It is generally 50 ohms or 70 ohms, so the impedance transformation must be achieved through follower BG2. Figure 2 is a bootstrap follower. Its characteristics are: 1. Bootstrap because the lower end potential of R3 rises with the upper end potential. It rises, so it is called self-boosting. The bootstrap effect makes the AC voltage drop across R3 zero. Therefore, for AC, R3 is equivalent to an open circuit, thus avoiding the defect of the bias circuit lowering the input impedance. 2. Input. High impedance In order to maximize the effective input impedance of the transistor, BG1 and BG2 are used to form a composite tube circuit. At this time, β=β1β2 greatly increases the total input impedance because the input impedance Ri=Rbe+(1+β)Reo of this circuit. The input impedance is 2 megohms. Figure 3 is a series-connected follower. Its characteristics are: (1) Similar to Figure 2, the AC voltage at both ends of R4 has a bootstrap effect; (2) BG2 adopts a common base connection method, so that Ic2 has a constant current function, and the AC impedance RAB of points A and B is greatly increased, thereby increasing the input impedance of the follower. Figure 4 is a complementary follower. The characteristics of the circuit are: (1) Since the two transistors take turns to supply. Load current, so the power consumption of each tube is only about (12-20)% of the output power, and the efficiency is high; (2) Both triodes output from the emitter, and their output impedance is basically the same, so the output waveform is positive, Negative half-wave symmetry; (3) Since the input signal is coupled to the base of the transistor through BG3 or BG4, both AC and DC signals can follow the following range of about ±5 volts.