I cannot afford a function generator and oscope right now, so I'm trying to learn analog electronics with Spice (MacSpice, to be specific). Below is the Spice model for a simple emitter-follower circuit. Vcc is 15V, and the input at the base is a 5V 60Hz sine wave with a 0V DC offset. R1 sits between the transistor base and the sine input, and R2 sits between the transistor emitter and ground.
The output at the emitter has me a bit puzzled and I would appreciate it if anyone has an explanation for the behavior.
* Simple Emitter Follower
Vcc 3 0 DC 15
Vin 1 0 SIN(0 5 60) dc 0
R1 1 2 270
R2 4 0 3.3k
Q1 3 2 4 generic
.model generic npn
.control
tran 1ms 60ms
plot v(1) v(4)
.endc
.end
The output of the transient analysis plot command is below.
I was surprised by this output. As expected, there is a slight voltage drop in the amplitude due to the base-emitter junction of the NPN. However, the output is clipped and does not swing below zero. What would I need to do to catch the negative part of the sine wave?
Answer
You need to connect R2 to a negative supply voltage with enough headroom for your signal, or bias the input waveform so that it doesn't clip.
The output voltage an emitter-follower is usually V_out = V_in - 0.7V, but there are bounds on where this function works. For the expected behavior, VCC + 0.7V > V_in > VEE + 0.7V must be maintained, where VEE is your negative supply. In your case, VEE is equal to 0V since you don't have a negative supply. When your input voltage swings below 0.7V, the transistor turns off, and your output stays at the negative voltage rail.
Modify these lines to add in a negative power supply.
R2 4 5 3.3k
Vee 0 5 DC 15
Modify this line to bias the sine wave for the existing circuit.
Vin 1 0 SIN(7.5 5 60) dc 0
No comments:
Post a Comment