Analysis of a Dual Bi-Polar Transistor Oscillator Circuit

amountdollElectronics - Devices

Nov 2, 2013 (3 years and 5 months ago)


Analysis of
a Dual

Polar Transistor Oscillator Circuit

Shawn Lam

Department of Applied Physics

Appalachian State University, Boone, NC


On December 15th 2004 a free
running mutivibrator was built and analyzed in order to
trace the propert
ies of the circuit. This circuit was composed of two PNP transistors as
well as four resistors and two capacitors. When a voltage is applied across the circuit two
LEDs blink alternatively back and forth governed by the RC discharge rate of the system.
After analysis with an oscilloscope a timing diagram was made in order to see the voltage
changes at different parts of the circuit. It was shown that the voltage at the base of the
PNP transistor was held above the threshold
voltage, V

0.6, until the c
apacitor attached
to it expels all of its charge to ground. Once the voltage at the base of the transistor falls
below V

0.6 volts the transistor will again turn on allowing a current avalanche through
the LED
attached to the collector thus lighting it.

While the transistor is discharging
it in
turn charges the second capacitor thus driving the base current of the second transistor
over its threshold voltage (almost twice what is needed). The routine repeats over and
over en infinitum.

1. Introduct

A free
running multi
vibrator circuit was built from schematics in a basic electronics
. This circuit oscillates a current avalanche alternatively between two LEDs so that
they blink back and forth. Upon initial examination the circuit is not
obvious and
deserved further analysis and investigation. This paper is meant to report the analysis
process and results as a teaching tool for analog electronics. This circuit utilizes many of
the key analog components used and studied in an introductory

analog class. Beyond the
integrated circuitry of most digital components understanding this circuit involves
knowing the underlying physics involved in these basic components and how they react
to one another.

Experimental Circuit

Figure 2.1

Diagram of free
running multivibrator used in experiment

Figure 2.1 shows a schematic layout of the circuit analyzed for the experiment. Two
10μF polar capacitors were used instead of the 22μF caps shown in the figu
3906 transistors were used as well as two
220 ohm and two 100 kohm resistors. The
symmetry of the circuit is evident by looking at the schematics.
The circuit was first built
on a pencil box and then later assembled on a
board (see figur
e 2.2)

Figure 2.2

Actual circuit blinking alternatively between the red and green LEDs

3. Circuit Analysis

An oscilloscope was used to look at the timing of the circuit by examining the voltages
at the base and collector of transistor Q
2 (see figure 3.1)
Surprisingly, the voltages at the

gure 3.1


Timing diagram
for the base and collector of Q2

base, event 1 (E1)

reached almost double of the driving voltage (+ 6V). This gave a clue
as to how the circuit worked. If the circui
t was frozen at t = 0, shown in figure 3.2, and

Figure 3.2

(t = 0) Transistor Q2 is turned on as is LED 1 (Red). Voltages at different points in the circuit
are shown.

he voltages recorded

it can be seen that transistor Q2 is
as is
. This
voltage of V at the collector of Q2 also sends a voltage of V to C1 thus driving the
voltage on the other side of C1, previously ~V
0.6, up to
0.6. As soon as this
happens it in turn shuts off Q1 by raising the voltage over the threshold

voltage of V
As soon as
transistor Q1 is turned off the charge built up on C1 (on Q1's side) starts to
discharge through R4 with an RC time constant:

. This
discharge governs the blink rate given by the exponential discharge
of a capacitor in
series with a resister:

(Equation 1)

Note the exponential decay of event 1 (E1) in figure 3.1 where the voltage jumps up to
0.6 and then discharges down to ~V
0.6. If V

= 2V and RC = 1 the time it takes for

the voltage to drop below the threshold, V
0.6 is given by equation 2:

(Equation 2)

This is the time that any one LED
, say LED1,

will stay lit

This is also

the time it takes
until the

, Q1,

switches on
. This i
n turn boosts t
he reference voltage of
the other transistor's base capacitor
, Q1,

thus shutting it off (see figure 3.3). The resistors
R1 and R2 only serve to protect the

Figure 3.3

(t = .69 sec) Transistor Q2 is turned off as is LED 1 (Red). Tran
sistor Q1 is now turned on as
is LED 2 (Green)

LEDs from excess current and do not play a role in the timing of the circuit.

Event 2 on figure 3.1 shows a sma
ll spike at the beginning of the voltage increase cycle.
This is indicative of the base cap
of the other transistor being charged to V. The voltage
remains relatively constant

e the transistor is switched on, and the current
avalanche flows through the LED,
and is set be V and R2:

(Equation 4)

so that the

power delivered to the LED is:

(Equation 5)

4. Conclusions

The free
running multivibrating circuit built for analysis purposes

be a very useful
tool in analog circuitry. The pulse signals are very predictable and can be u
sed for a
variety of applications such as toys, a timing circuit, etc. By changing the capacitors the
pulse with can be modulated to a desired rate. This circuit is similar to a 555 timer in
digital electronics but is a more useful learning tool for anal
og. It could be used as a
teaching tool for an introductory electronics class. Thinking through the processes
involved helps the student gain a better understanding of the physic

involved in more
complicated (but more common) integrated circuits such as

the 555 timer.


I would like to thank Dr. Adrian Daw for all his help analyzing this circuit and teaching
me the subject of analog electronics.


(1) Mims, Forrest, Getting Started in Electronics, 1st edition,
pg. 113, Radio Shack, 1993


Hayes, Thomas, Horowitz, Paul, The Art of Electronics, Cambridge University Press,
New York, 1989

Smith, Ralph, Electronics Circuits and Devices, 3rd edition, Wiley, New York, 1987

Tocci, Ronald, Widmer, Neal, Moss, Gregory, Digital Systems Principles and
Applications, 9th edition, Pearson Prentice Hall, Columbus, 2004