Design Guidelines for Small Signal Amplifier Circuits
How to choose the right component values
By Dave Cline
To understand how to correctly implement a transistor as a functional part of a circuit is extremely crucial in learning the basics of electronics. The small signal audio amplifier is perhaps the most common practical circuit first introduced to the student of electronics. It does serve as the perfect example for illustrating basic transistor operations. So what better way to show how a single transistor can be used in a real world application than by showing you step-by-step how to design such a circuit. The amplifier circuit that we are going to design is called a "common emitter amplifier" built around a bipolar junction NPN transistor - more specifically the popular PN2222A transistor (click here for specifications). The final circuit presented at the end of this document can be used as a microphone pre-amplifier or any application that requires small signal amplification.
Before we begin, it is important to note the difference between Conventional Current and Electron Flow. These hold opposite viewpoints in the direction that electric current flows. Conventional Current assumes that positive-charge carriers flow from the positive terminal of the current source, through the circuit and into the negative terminal. Electron Flow describes electric current as the flow of negative-charge carriers flowing from the negative terminal, through the circuit and into the positive terminal. In wires and most electronic components, electric current is actually the flow of negative-charge carriers called electrons. Therefore, Electron Flow describes the direction that electric current actually flows. Nevertheless, Conventional Current is used more frequently by universities and by the electronics industry, for which you can blame the history of electrical science. Back in the early days of electricity, the exact nature of current flow was not understood, so it was assumed that current flowed from the positive terminal to the negative. It really does not make any difference which direction anyone thinks that current flows, as long as they are consistent. It has always been my personal preference to describe current flow as Electron Flow. Therefore, all current arrows in the following diagrams will point in the Electron Flow direction. I also refer "ground" as the the negative side of the current source and "voltage" as the positive side. All voltages are measured with respect to ground. Since the emitter lead is connected to ground, both the input and output is in reference to the emitter. Hence the name "common emitter."
This document assumes that you are already well aquainted with the transistor. Even so, it would not hurt to go over some of the basic principles. Remember that a small current flowing from emitter to base (known as base current) causes a much greater current to flow from emitter to collector (known as collector current). In order for base current to begin flowing, the voltage applied across the base-emitter junction must reach the "turn on" threshold voltage. When the threshold voltage (or greater) is applied at the base, the base-emitter junction is said to be forward biased.
Setting the baseline voltage
© 2007 Dave Cline. All Rights Reserved.
The threshold voltage for silicon transistors is usually between .6 to .7 volts (for the sake of convenience, we'll pick .7 volts). No matter how much voltage is applied at the base of the transistor, the voltage drop across the base-emitter junction will always be .7 volts (assuming that there is at least .7 volts or more present at the base). Since the voltage across the base-emitter junction is unable to change beyond the threshold voltage, there is no way for the transistor to know how much voltage is being applied at the base. This being the case, how can a transistor amplify an input signal that consists of varying changes in voltage? A transistor can respond to changes in voltages applied at the base because as the voltage changes, the current changes along with it. An increase in voltage applied at the base of the transistor will cause an increase in current flowing across the base-emitter junction, thereby increasing the collector current. The transistor does not care that the voltage has been increased - it only feels the resulting increase in current. So, strictly speaking, you could say that a transistor is a current amplifier - not a voltage amplifier. However, a transistor can be "tricked" into emulating a voltage amplifier with a little help from a resistor.
According to ohm's law, any change in current flowing through a resistor must reflect a change in voltage developed across the resistor. If we connect a resistor (called the load resistor) between the collector and supply voltage, we can convert changes in collector current into corresponding changes in voltage. So as the signal voltage at the base increases, the collector current is increased, thereby causing the voltage developed across the resistor to increase (known as the load voltage). However, an increase in current has the opposite effect on the transistor. As the collector current goes up, the voltage betweem and collector and ground (known as the collector voltage) goes down. That is because an increase in collector current "neutralizes" a portion of the voltage supplied to the collector from the load resistor. In other words, as the signal voltage is increased, the collector voltage is decreased. Now, if the signal voltage is decreased, there will be a corresponding decrease in collector current. This decrease in current pulls the collector further away from ground and brings it closer to the supply voltage, thereby increasing the collector voltage. An increase in collector voltage is simultaneously followed by a decrease in load voltage. You see, the total combined voltage dropped from both the load resistor and transistor will always add up to be equal to the supply voltage. If one has more, the other has to have less to maintain the balance.
The voltage that is dropped across the load resistor is not in reference to ground (the voltage is dropped between the supply voltage and where the resistor connects to the collector), but the collector voltage is in reference to ground (the voltage is dropped between the collector and ground). Therefore, the output follows changes in collector voltage, not the load voltage. Since an increase in input signal voltage produces a corresponding drop in output voltage and vice versa, the circuit operates as an inverting voltage amplifier.
A resistor placed between the collector lead and the power supply provides voltage to the transistor, but at the same time allows room for the collector voltage to vary according to the input signal. If the collector was tied directly to the supply voltage, the collector voltage would be unable to change below the supply voltage.
An increase in collector current raises the load voltage while lowering the collector voltage. Likewise, a decrease in collector current lowers the load voltage while rasing the collector voltage. The sum of the load and collector voltage must equal the supply voltage.