# R V dt di RC dt di τ dt di RC τ τ i τ τ i −= + + + =

```JS ENGINEERING : 3C2 ELECTRONIC ENGINEERING I
TUTORIAL PROBLEM SHEET No. 6
2011-12
THE BIPOLAR TRANSISTOR INVERTER : DYNAMIC CHARACTERISTICS
Q1.
In the single transistor inverter circuit shown below, the input voltage is switched abruptly
from Vcc to 0V. The base current as obtained from the Charge Control equations governing
turn-off of the transistor is given as:
iB =
VBE ON
di
di
di
τF
τ
i C + F C BC R C C + τ F C + C BC R C C = −
τ BF
τ BF
dt
dt
dt
RB
Solve this equation to obtain an expression for the collector current and the output voltage of
the inverter during turn-off of the transistor. Hence evaluate the 10% - 90% rise-time of the
output voltage of the inverter if τF = 0.2ns, βF = 50, RC = 1kΩ, RB = 10kΩ, CBC = 0.5pF and
VCC = 5V.
VCC
RC
RB
V
CC
V
0V
Q2.
Vo
i
In the capacitively-loaded single-transistor inverter shown below, the input voltage is
switched abruptly from VCC to 0V at time t = 0. If the transistor is assumed to act as an ideal
switch, sketch the waveform of the output voltage as a function of time and establish an
expression for this voltage. Hence, derive an expression for the low-to-high propagation
delay of the inverter in terms of the circuit parameters.
Determine the maximum tolerable load capacitance if this propagation delay is not to exceed
10ns when βF = 50, RC = 1kΩ, RB = 10kΩ and VCC = 5V.
VCC
RC
V
RB
CC
V
0V
CL
i
Vo
```