# homework assistance

• posted
----------------------------------------------------------------------------
------------ 5V
| |
resistor (R2-1K) |
| resistor R4(1k)
|---------------- |-----------O
| | |
O--resistor---Transistor (T1) --------------resistor
R3(33k)---Transistor (T2)
(R1 33k) | |
Vin | | Vout
| |
O---------------------------------------------------------------------------
--------O
Ground
circuit above, could anyone drop me
some hints
?
Figure 2 shows two bipolar switches (inverters) in cascade, The first is
driven by a perfect square-wave
input signal with zero source impedance, which switches between 0 to 5V.
Swithcing time for the
first switch are ton1 = 82ns and toff = 454ns. For a second switch ton2 =
85ns and toff2 = 639ns. The large
variation in toff is mainly due to the differences in Ib(off) of the
switches. Each transistor has
Vce(sat) = 0.2V and Vbe(on) = 0.7V
a) Draw an equivalent circuit that models the input section of the first
switch and its signal source
when Ib1(off) flows. Calculate Ib1(off)
b) Now Draw an equivalent circuit that models the input section of the
second switch and its signal source when
Ib2(off) flows. Hence calculate Ib2(off)
c) For the first stage, td1 = 20ns, tf1 = 62ns, ts1 = 77ns and tr1 = 377ns.
The second stage has td2 = 20ns
tf2 = 65ns, ts2 = 102ns and tr2 = 537ns. To determine the switching delays
at the output of the combination
(ie at the collector of T2) it can be assumed that T2 starts to switch when
the collector of T1 reaches 2.5V
Sketch waveforms of the input signal, the voltage at the collector of T1 and
the output voltage to illustrate
how the above switching times apply; indicate the values on the waveforms.
I have been seeking for the answer for the last few weeks and I don't still
don't understand. Although it is
homework, I'd like to have to hints to start off with. Thanks
Jack