- [Instructor] Transistors have two
fundamental applications,
building electric controlled switches
and amplifying electrical signals.
Now, although I frequently use transistors as switches,
I'll admit I rarely use individual transistors
to build amplifiers
because in day to day circuit building,
it's a lot easier for me to use another type
of active component called an operational amplifier,
which I'll cover later in this course.
That said, even if you never build an amplifier
out of BJTs, it's helpful to understand how they work
because transistors are a fundamental component
behind active circuits.
An amplifier is an electronic device
that can increase the power of a signal
meaning the signal coming out of an amplifier
has more power than the signal that was put into it.
Now, an amplifier just can't magically
create that power out of nothing.
That's not physically possible.
So we'll need to get its power from somewhere else.
The amplifier works by drawing electrical power
from a separate source
and then uses that power to increase the amplitude
of a signal's voltage or current.
There's a wide variety of amplifier circuit designs
that use BJTs but they're all based
on one of three basic configurations,
a common collector, common base, and common emitter.
The name of each of those configurations
indicates which of the three BJT terminals
is tied to a common voltage
while the other two terminals
serve as the amplifier's input and output.
Each configuration can provide different amounts
of voltage or current gain
and have differing input and output
impedance characteristics.
In the common collector configuration,
the collector terminal is tied to a common fixed voltage
while the input signal is sent to the base terminal
and the output comes from the emitter.
For simplicity,
I'm just showing the BJT here to explain the concept,
but in practice, all of these configurations
will require additional components not shown here
to properly bias the transistor
which means put the transistor in the correct DC voltage
or current operating conditions
to keep it in the active region
so it can be used to amplify an AC input signal.
The common collector configuration is often called
an emitter follower or a voltage follower
because the output voltage from the emitter
will roughly match or follow
the voltage of the input.
This circuit doesn't provide any voltage gain.
However, the common collector
can provide a lot of current gain
which makes it great for use as a voltage buffer
which is a circuit that provides
as much current as the load connected to its output needs
while keeping its output voltage
the same as the input voltage signal.
A voltage buffer is useful
for connecting a signal source
with a high output impedance
to a load circuit that has a low input impedance,
to prevent the load
from affecting the behavior of the source.
The common collector configuration
has a high input impedance
and a low output impedance
which enables it to bridge the impedance difference
between the circuit elements.
The common base configuration
is probably the least common of the three.
In it, the base terminal is tied to a common reference,
the emitter terminal receives the input signal,
and the collector terminal produces the output.
The common base configuration has a low input impedance
and a high output impedance
which makes it useful as a current buffer.
It can provide a reasonable amount of voltage gain
while keeping the output current roughly equal
to the input current.
The third and final transistor configuration
is the common emitter.
As the name implies,
the emitter is tied to a common fixed voltage.
The input signal is applied to the base,
and the output is produced at the emitter terminal.
The common emitter circuit
is the most popular of the three transistor arrangements
because it's well suited for amplifying voltages.
And, it's also capable of providing current gain.
The common emitter does have one quirk though
which is that its output signal
will be an inverted or flipped version
of the input signal.
Since the common emitter is the most popular configuration,
let's dive a little deeper
to see what goes into making it work.
I won't be showing you how to design transistor
amplifiers from scratch,
that's a complex topic and well beyond
the scope of this course,
but I will step through the basic workings
of a common emitter amplifier
to give you a high level understanding
of how it works.
The first thing I need to do
to use this BJT as an amplifier
is bias the base to emitter junction
to turn the transistor on
so it'll be operating in its active region.
One common way to do that
is to use two resistors
to create a voltage divider
between the power supply and ground
to produce the required voltage
to bias the base terminal.
In the example circuit shown here,
I'll use a 10 kilo-ohm and a 100 kilo-ohm resistor
to divide down the voltage from a 12 volt power supply.
The combination of those two resistors
will produce around 1.1 volts at the middle
which is connected to the base terminal
and is a high enough voltage
to turn on my transistor.
A small amount of current will flow down
from the 12 volt power supply
through the 100 kilo-ohm resistor
and into the base.
When the transistor is turned on,
there will be a voltage drop
of around 0.7 volts from the base to emitter
which leaves 0.4 volts to account for
across the bottom resistor
that's connected to the emitter terminal.
Now, to use this transistor as an amplifier,
I want to keep it operating in the active region
where the amount of base and collector current
has a linear relationship
that follows the transistor's gain factor, beta.
If I make the bottom resistor connected
to the emitter 1 kilo-ohm,
then according to ohm's law,
0.4 volts across that 1 kilo-ohm resistor
will draw just 0.4 milliamps of current from the emitter.
The amount of current that flows into the collector
will be slightly less than the current from the emitter.
The exact amount will depend on this transistor's
individual beta value.
But to keep this example simple,
for now I'll pretend like the collector
and emitter currents are the same.
Both have 0.4 milliamps flowing through them.
If I make the top resistor
that's connected to the collector terminal
a 10 kilo-ohm resistor,
that 0.4 milliamps flowing through it
will create a voltage drop of 4 volts.
That voltage drop connected
to the 12 volt power supply voltage
means the output of this circuit
will be at 8 volts.
The values shown here represent
the amplifier's quiescent or quiet state.
The circuit is inactive, it's not driving any output load
and its input signal isn't changing.
Now that's not very exciting
so let's connect the input to an AC signal source.
That AC input signal
could come from any number of sources
such as the audio signal
coming out of an MP3 player.
I want to make sure that the input signal
only includes AC components
because if the input signal has any DC offset
that could interfere with my carefully
crafted DC biasing voltage
created by the voltage divider.
So to protect against that,
I've included a capacitor in series
with the AC signal source
and the transistor's base terminal.
This is called a coupling capacitor
and it acts as a high pass filter
to block any DC energy
from the incoming signal.
Just the AC signal components will pass through
and then they'll be added to the DC bias voltage
that I created with the voltage divider on the other side.
I'll need to choose a capacitor value
that will allow the AC frequencies that I care about
on my signal to pass through.
For the resistor value shown here,
using a 1 microfarad capacitor
should produce a high pass filter
that will let most frequencies
in the audio range through.
Additionally, if I want to connect the output
from this amplifier to a device
that's expecting an AC signal,
I'll need to include a coupling capacitor on the output
to remove the DC offset from the amplifier's output
before sending that signal on
to whatever load I have connected downstream.
The quiescent voltage at the transistor's collector terminal
is 8 volts
but the coupling capacitor filters out that DC energy
so the output will be centered at 0 volts.
Now that I have all of the components in place,
let's look at what happens
when I give this circuit a sine wave as the input signal.
Assuming the frequency of the sine wave
is within the pass band of the coupling capacitor
on the front,
when the sine wave swings high
to a semi amplitude of 100 millivolts,
that AC energy will pass through the coupling capacitor
and raise the voltage at the transistor's base
by 100 millivolts
bringing it up to 1.2 volts.
Since the 0.7 volt drop
from the base to emitter remains constant,
that extra 100 millivolts will get dropped
across the bottom emitter resistor
which in turn increases the current through the transistor
from 0.4 to 0.5 milliamps.
That increases the current through
the top 10 kilo-ohm resistor
making the voltage drop across it increase
from 4 to 5 volts.
The larger drop of 5 volts across the top
collector resistor means the output voltage
at the collector terminal
will decrease by 1 volt
down from 8 volts to only 7 volts.
That 1 volt drop was part of a changing AC signal
so it passes through the output coupling capacitor
making the output voltage at that moment in time
negative 1 volt.
As you can see, there's quite a chain of events that occur
between changes to the input
and changes to the output signal.
When the input sine wave swings down
to negative 100 millivolts,
that causes all of those values to shift
in the other direction.
The voltage at the transistor's base
will drop to 1 volt
which reduces the voltage drop
across the bottom emitter resistor
to only 0.3 volts.
The current through the transistor
will drop to 3 milliamps
which creates less of a voltage drop
across the top resistor connected to the collector.
And that smaller voltage drop of just 3 volts
means the output voltage directly at the collector terminal
will be 9 volts.
And finally, after passing through
the output coupling capacitor to remove DC bias,
the corresponding output signal at that point in time
will be positive 1 volt.
So, when the input voltage was at positive 100 millivolts,
the output was negative 1 volt.
And when the input voltage was negative 100 millivolts,
the output was positive 1 volt.
Since gain is calculated as the output voltage
over the input voltage,
this amplifier circuit
has a voltage gain of negative 10.
Now, I made a lot of simplifications
throughout that example
to focus on just the basic concepts at play.
As you can see, there are a lot of interconnected pieces
that make this circuit function
which is why amplifier design
is an art of its own.
If you want to learn more
about how to build your own amplifier circuits,
then I recommend diving into the reference sources
I've listed in the exercise files.