AP Phys 2 Transistor HW
Access the Circuit Simulator at http://www.falstad.com/circuit/
The files needed are available as attachments to this homework on Whipple Hill. Copy them
to your local hard drive: emit_follow.txt, comm_collect.txt
1. Open the file emit_follow.txt in any
word processor. Select and copy the
entire text. Under the Circuit
Simulator’s File menu, choose Import
and paste the copied text into the Import
text box. Click ‘Import’ and close the
text box.
R1
R2
The circuit shown at right, which is an
“emitter-follower” amplifier, should
appear. The output is taken from the
emitter of the NPN transistor.
R3
Be sure that the scope display below the
circuit has the source voltage and
current on the left and the output
voltage on the right.
a. The output voltage, measured across the R3 load resistor, never goes negative. Why?
I
V
b. Gain can be calculated on the basis of current,   e or voltage,   out . Determine
Ib
Vin
both gains for this circuit. Determine the power gain (power = V I) for this circuit.
c. Create a graph of current gain as you increase the resistance R2 up to 4800. How does
the appearance of the output begin to differ from the shape of the input as gain increases?
(this is known as distortion – a bad thing).
2. Open the file comm_collect.txt; the following circuit should be displayed. The output is
now taken from the collector of the NPN transistor.
a. Determine both the gains  
Ie
V
and   out for this circuit. Determine the power gain
Ib
Vin
(power = V I) for this circuit.
b. What happens to the output as you reduce the +20V supply voltage? This is sometimes
called ‘DC bias;’ what is the value of DC bias that produces an output that is symmetric
around V=0? Note: you can see what is happening if you make the upper 10k resistor
visible in the scope.
c. The 110k and 10k on the base side of the transistor form a voltage divider. What are
the roles of the 10k and 1k resistors? Determine how the output of the circuit responds
when you change their values. Is the 10:1 ratio significant?
3. The symbols for the commonly used basic logic gates are available on hyperphysics,
http://hyperphysics.phy-astr.gsu.edu
/HBASE/electronic/diglog.html#c1. In the diagrams below, x means multiply and + means
add (but you already knew that).
Determine the truth table for the logic circuit shown below (0=false, 1=true). There is no
connection where wires cross.
a. Two inputs
(A, B)
b. Three inputs
(A, B, C)
Make up a logical statement (A and B or not C, etc) that fits this circuit’s output. Apply that
logical outcome to illustrate an everyday situation.