Join Barron Stone for an in-depth discussion in this video Use a BJT as an amplifier, part of Electronics Foundations: Semiconductor Devices.
- [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.
- Semiconductor materials
- Diode applications
- Rectifying a signal
- Detecting the signal peak
- Protecting against large signals, reverse current, and flyback voltage
- Special purpose zener diodes, Schottky diodes, and photodiodes
- NPN and PNP bipolar junction transistors
- Using a BJT as a switch
- Field effect transistors
- Differences between BJTs and MOSFETs
- Operational amplifiers
- Op-amp applications
- Comparing signals
- Buffering signals
- Amplifying signals
- Filtering signals
- Combining signals