The
transistor is simply amplifying the current in the base circuit and every
transistor has a maximum amplifying factor. If this value is say 250 and the
current required by the globe for full brightness is 250mA, we have to deliver
1mA into the base. If the resistor is chosen so that it delivers less than
this, the globe will be dull.
To create a circuit to
do this we can select different-value resistors and try them one at a time or
add a variable resistor in series with the base resistor. (You cannot leave out
the original base resistor and replace it with a variable resistor as the
transistor will be damaged when the variable resistor is reduced to a low value
when the shaft is rotated in one direction).
A variable
resistor is called a POTENTIOMETER. To see the effect of adding extra resistance
to the base circuit, click: "View."
Question 57: Name the three leads of a transistor.
Ans: Collector, base, emitter.
Question 58: Name the input lead of a transistor:
Ans: base
Question 59: The globe will be bright when the
resistance of the potentiometer is high/low?
Ans: low
Question 60: What is the direction of current
flow?
Ans:
From positive to negative
Now we will add a
capacitor to the circuit to show the effect it has on the brightness of the
globe, when it is connected for the first time. We have already discussed how it
works and the animation shows the globe starting with full brightness when the
uncharged capacitor is connected to the circuit. The reason is the uncharged
capacitor "looks like" a very low-value resistor to the rest of the
circuit and that's exactly like the potentiometer turned to
low-resistance.
As the capacitor charges, the globe dims and finally goes out. It goes out
because the capacitor does not pass DC. It only allows a current to flow when it
is charging.
There
is one thing you have to know about the animation above. The effect shown only
works the FIRST TIME you close the switch because the capacitor is uncharged at
the beginning. If the switch is opened, the capacitor will be fully charged and
when the switch is closed for the second time, the globe will remain unlit. You
have to discharge the capacitor for the circuit to work the second time.
Discharging
the capacitor means to connect its two ends together. This can be done
mechanically with a switch or by electronic means. If the two ends are connected
together directly, it is called SHORT-CIRCUITING the capacitor. If the two ends
are connected via a resistor or some other device, it is called DISCHARGING the
capacitor. If you want the capacitor to repeat the effect shown above, it must be discharged between cycles. This
is one of the hardest things to do in electronics, but it must be done (even
partially discharged) for the capacitor to work in the next cycle. More about
this later.
There are two more things you have to
know about a transistor to understand how it works. These are
"characteristics" of a transistor (just like the voltage drop across a
LED or the voltage drop across a diode) and cannot be changed. They are the
"secret" to knowing how a circuit works.
The
two characteristics are :
1. The base "turn-on" voltage, and
2. The collector-emitter voltage drop.
In the
animations above we have shown the transistor "turning on" when a
voltage is supplied to the base and a current is delivered that is sufficient to
turn the transistor on.
There is one point we
haven't mentioned. The voltage on the base must be 0.7v for the transistor to
turn on. If the voltage supplied to the base is below 0.6v, the transistor DOES
NOT TURN ON AT ALL. This is an amazing characteristic and we can show it in the following animation. There is a very small gap between
0.6v and 0.7v where the transistor changes from "not turned on all
all" to "fully turned on" and maybe we can say the transistor is
partially turned on at 0.65v.
For this
experiment we use a voltage divider resistor. This is a potentiometer connected
between positive and negative rails and as the shaft is rotated, the output will
vary from 0v, to full rail voltage. We only want a voltage between 0.6v and 0.7v
and the pot (potentiometer / voltage divider resistor) has to be turned very
carefully.
The
second characteristic of the transistor is the voltage between collector and
emitter when the transistor is fully turned on. This state is called
"SATURATION" the voltage is approximately 0.3v. The animation below
shows a multimeter connected between the collector and negative (0v) rail. When
the transistor is off, the voltage on the collector will be rail voltage (in
this case 6v). When the transistor is partly turned on, the voltage will be
about 3v.
There's
another way to see what is happening between the transistor and globe. You can
think of the two components as resistors. In the first frame, when the
transistor is not conducting, it can be thought of as a high value resistor
connected to a low value resistor (the globe). This produces a high voltage
(rail voltage) at their join.
In the second
frame, the resistance of the transistor is approx equal to that of the globe and
the voltage at their join is about half-rail voltage. In the third frame the
resistance of the transistor is considerably less than the globe and the voltage
across the transistor is very small. This is shown in the animation below:
Now
we come to combining the facts we have learnt and see why the "turn-on
voltage for the base" and "collector-emitter voltage" is so
important for the operation of some transistor circuits. There are basically two
types of transistor circuits. DIGITAL and ANALOGUE. Digital circuits work on the
principle of the transistor being either fully ON or OFF. Analogue
circuits allow the transistor to be partially turned on. Digital circuits are
used in computers for storing information and transferring it from one area to
another. Some oscillators, such as the multivibrator, are digital in operation
as the transistors are either ON or OFF.
Analogue
circuits are used for audio and some types oscillators such as sine-wave
oscillators. The next circuit demonstrates how one transistor (Q1) can
control another transistor (Q2). It is fortunate Vbase
turn-on voltage = 0.7v and Vcollector-emitter = 0.3v
otherwise many transistor circuits would not work!
In
the circuit, Q1 is turned on and off via the potentiometer as shown in the animation above. We know the collector voltage
will change from rail voltage when turned off to 0.3v when turned on. The base
of Q2 is connected directly across the collector-emitter
terminals and when Q1 is turned on the voltage across it
is lower than the 0.7v required to turn on Q2 and thus Q2
is turned OFF. If you are constructing this circuit, it will be difficult to see
the change in voltage on the collector of Q1 because the
base of Q2 is connected directly to it and the voltage
will only rise to 0.7v when Q1 is off. Note also, that the
globe is off when the voltage on the base of Q1 increases and on when the
voltage on the base of Q1 is zero. This is the opposite to the animation above
because Q2 is INVERTING THE SIGNAL. In other words, Q2 causes INVERSION.
Question
61: When the voltage on the base is 0.6v, the transistor is: ON, Off?
Ans:
The transistor is OFF.
Question 62: To
turn a transistor on FULLY, the voltage on the base must be:
Ans:
0.7v
Question 63: When a transistor is
turned-on, the voltage between the collector and base is :
Ans:
0.3v
ADDING
A CAPACITOR
There is just one more
thing we have to cover before we can go into explaining how various different
circuits work. It's the secret of discharging a capacitor so an oscillating
waveform can be passed to a transistor. We say above, a capacitor will only
operate the first time in an oscillating circuit. Once it is charged, it does
not provide the required effect. The solution is to discharge it so it can be
ready for the next cycle. To understand how this is done, we need three more
animations.
The first animation shows the
capacitor charging when the voltage on the left-hand side rises. When the
voltage falls, the capacitor is fully (or nearly fully) charged and you can
think of it as a tiny battery of say 5v (if the supply to the circuit is about
6v).
Replace the capacitor with the 5v battery
and now turn the potentiometer to take the left-hand side down to the 0v rail. What happens to the
right-hand side of the battery? It drops by 5v and actually goes 5v below the 0v
rail. The base sees this -5v and and as we explained above, the transistor does
not turn on unless a +0.65v is present on the base. This means the 5v battery
will not discharge into the base of the transistor and it will remain charged.
There
is no point is raising the left-hand side of the battery (capacitor) to the
positive rail as the capacitor is charged and will not turn on the transistor.
This is shown in the animation below:
The solution is to put a resistor between base and 0v rail to discharge the 5v
battery (capacitor). The capacitor (battery) does not have to fully
discharge but it best to discharge it as much as possible. This will depend on
the value of the capacitor and resistor.
Now
you can see how a capacitor puts negative voltage on the base of a transistor.
It would be almost impossible to show this effect without the aid of
animation.
The animation above is showing three
things. The potentiometer moving up and down is exactly the same as a waveform
entering the circuit. We saw this effect above and performed it with the pointer
of the mouse. The waveform could be a sinewave or audio. Any oscillating
waveform will create the same effect with the capacitor. The length of the
battery represents the charge in (or on) the capacitor and you can see how the
capacitor fills and empties as the wave enters the circuit. The transistor only
turns on when the wave is 0.65v higher than the voltage in the capacitor
and this might occur when the waveform is 3v or 4v. It all depends on the size
of the capacitor and base resistor.
When the wave is
falling, the charge in the capacitor is removed by the base resistor and this
continues until the wave turns the transistor on again. If the incoming
frequency is 1,000Hz (1kHz), the action is occurring 1,000 times per second. If
the frequency is 100MHz, it is occurring 100,000,000 times per second!
Question
64: How does the capacitor in the animation above, get charged?
Ans:
The incoming waveform is the "supply" and the capacitor is charged via
the base resistor and the 0V rail completes the circuit.
Question
65: Give two names for the waveform produced by the potentiometer in the
animation above, moving up and down:
Ans:
AC sinewave,
Question 66:
Explain why a capacitor passes AC:
Ans:
Electricity flows in and out a capacitor when a waveform is delivered to one of
its leads and the other components in the circuit see this as exactly the same
as the flow of electricity.
Question 67:
How does the capacitor in the animation above gets discharged?
Ans:
Via the base resistor and through the 0v rail.
Question
68: When the waveform in the animation above starts to fall, why does the
transistor turn off immediately?
Ans:
The waveform only has to fall 0.1v and the transistor changes from "FULLY
TURNED ON" to "NOT TURNED ON."
This
completes some of the most important concepts to understanding how a wide range
of transistor circuits work - especially oscillator circuits such as the
feedback amplifier and multivibrator. These are the next two circuits we will be
covering.
NEXT