Amplifiers

Most electronic circuits are amplifiers. You apply a voltage to the input of an amplifier and this causes another voltage to appear on the output. The voltage that gets modified by the amplifier is called the signal. We usually think of an amplifier as a circuit that increases the signal level. This is usually, but not always the case.

An Amplifier .

The generic symbol for an amplifier. The input is to the left and the output is to the right.

Amplifier configurations

There are three basic amplifier configurations. The names of these configurations are based on the amplifier's active element. For example, a vacuum tube's terminals are the cathode, the anode and the grid. The amplifier configurations would then be common-cathode, common-anode and common-grid. It the active element is a field-effect transistor, the configurations are called common-source, common-drain and common gate. We are going to use bipolar junction transistors for our examples below, so the names of the configurations are common-emitter, common-collector and common-base.

Common-Emitter Amplifier

In the following diagram, the transistor is connected between two separate circuits. To the left is the base circuit. The base circuit includes the input signal source. The input signal flows into the base of the transistor, out of the emitter and back to the source. To the right is the collector circuit. The collector circuit includes the power source (the battery in this example). The current flows into the collector, out of the emitter and back to the power source. Both circuits include the emitter; the emitter is common to both circuits. This is why it is called a common-emitter amplifier.

In this example, we are interested in how the voltage at the collector reacts to the base current. The output of the amplifier is the collector voltage (V_C). In complex amplifiers, it can be difficult to see the common connection at the emitter. Because of this, the best way to identify a common-emitter configuration is to see that output is at the collector (output at the collector = common-emitter).

Schematic diagrams often simplify amplifiers by not showing the power source or the lines that connect to it. These are replaced with symbols. The circle at the top of the circuit is connected to the power source. This is often labeled with a plus sign and the voltage. It is also often labeled as V_CC (The double C represents that this is at or near the collector of the transistor). The negative side of the power source connects to the circuit ground (the common connection) at the emitter of the transistor. This ground may or may not be connected to a true earth ground. When not connected to a true earth ground the common-ground symbol should be used (but don't count on it). The input source is also not shown below. Only an input connection is shown. The input source also connects to the circuit ground.

The resistor connected between V_CC and the collector (R_C) isolates the collector from the power supply. Without this resistor, the voltage at the collector would always be the same as the supply voltage. V_C is the voltage measured at the collector (referenced to ground). In the simple circuit above V_C and V_CE (the voltage measured between the collector and the emitter) are the same.

Increasing the base current causes the collector current to increase. The collector current flows from V_CC through R_C. Remember that when current flows through a resistor, there is always a lower voltage where conventional current exits the resistor than where the current enters the resistor. When there is no current flowing into the collector, and therefore no current flowing through R_C, there is no voltage difference across R_C. In that case, the collector voltage (V_C) will be the same as V_CC. When current is flowing, V_C will be less than V_CC.

Let's repeat that for emphasis and clarity. No base current means no collector current. No collector current means no current through R_C. No current through R_C means no voltage difference across R_C. Therefore, V_C will be the same as V_CC. When there is collector current, that current must flow through R_C. Then, there will be a voltage difference across R_C. V_C will be lower than V_CC. If you increase the collector current, V_C goes even lower. Increasing the base current increases the collector current. Therefore: When the base current goes up, the collector voltage goes down.

Increasing the base current increases the collector current, causing a drop in the collector voltage.

Think of it like sucking on a soda straw that you have mostly pinched off. The harder you suck, the lower the pressure in the side of the straw you are sucking on.

Once again, increasing the base current decreases the V_C. Decreasing the base current increases V_C. The collector voltage always goes in the opposite direction of the base current (the output is inverted compared to the input). If you put a sine wave at the input, the output will be 180 degrees out-of-phase with the input.

The output signal of a common-emitter amplifier is 180 degrees out-of-phase with the input.

Input Impedance

The base-to-emitter junction of the transistor has a low impedance. This means that BJT amplifiers take a relatively high current to operate compared to FETs and vacuum tubes. A common-emitter amplifier has a low input impedance. The input impedance depends on the base-to-emitter impedance of the particular transistor used in the amplifier.

Output Impedance

The collector circuit of our example is a parallel circuit (it has multiple current paths). Some of the current that passes through R_C goes into the collector of the transistor and the rest goes through the load. Since the output current flows through R_C, the output impedance of the amplifier is relatively high and is equal to the value of R_C.

Current flow in the collector circuit. The load current passes through RC

Voltage Gain

The base-to-emitter junction of a transistor acts as a diode. This diode is forward-biased during normal operation. As such, small changes in input voltage cause an exponential change in input current (see the characteristics of diodes). This causes an even greater change in collector current. As discussed above, the output voltage depends on the collector current that flows through RC. Increasing the value of R_C causes a greater voltage to be produced across R_C at a particular current. The voltage gain of this amplifier depends on the h_FE of the transistor and R_C. An increase in either the value of R_C or h_FE causes an increase in voltage gain.

Another thing that may be confusing is that the collector voltage goes down as the base current goes up. This may look like a loss rather than a gain. Concerning the amplifier gain, we don't consider whether the collector current goes up or down, but how much it goes up or down. When the base current changes a little, the collector voltage changes a lot. The collector voltage just goes the opposite direction to the base current. Take another look at the illustration where the input is a sine wave. The output is a bigger sine wave. The output is just inverted compared to the input; when the base current goes up, the collector voltage goes down and vice versa.

Current Amplification

The common-emitter amplifier can be used as a current amplifier. This is done by replacing R_C with the load. In the following circuit, RC has been replaced with a loudspeaker.

A Current Amplifier (actually a power amplifier)

When the load is parallel to the transistor, as in the previous circuit, the common-emitter amplifier acts as a voltage amplifier; the collector voltage is applied to the load. When the load is in series with the transistor, the amplifier acts as a current amplifier; the collector current goes through the load. In this simple common-emitter amplifier, the current gain is equal to the hFE of the transistor. This circuit produces power as it imparts motion to the air to create sound waves. Therefore, this is actually a power amplifier.

Saturation

 A transistor acts as a current-controlled variable resistor; increasing the base current decreases the resistance between the collector and the emitter and thus increases the collector current. At some point, this resistance reaches a lower limit; it can only go so low. When the transistor's resistance reaches this lower limit, increasing the base current will have no further effect on the collector current or the collector voltage regardless of the published value of h_FE. The circuit has reached the saturation point. Saturation is the point where increasing the input signal no longer causes a change in the output.

Here is an example of a circuit under normal operation:

This transistor has an hFE of 100. 50 μA applied to the base causes 5 mA of collector current (50 μA X 100 = 5mA). The collector resistor has a value of 1 k. With 5 mA flowing through it, it will have 5 volts across it. This is half of the supply voltage of 10 V. As per Kirchhoff's Voltage Law, the other 5 volts must be across the transistor. The circuit is operating normally.

If the base current reaches a certain value (that value depends on the h_FE of the transistor and the value of the collector resistor) the transistor hits its lower resistance limit (let's say 0 ohms for now, just to keep things simple). The transistor's resistance cannot decrease, the collector current, therefore, cannot increase and the entire supply voltage will be dropped across the collector resistor. The collector voltage (the output voltage) will be at 0 volts. Increasing the base current cannot make it go lower. The circuit has reached the saturation point.

This circuit has reached the saturation point.

This circuit, with a transistor hFE of 100 and a collector resistor of 1 k, saturates when the base current reaches 100 μA.

An Engineering Perspective

The transistor's resistance is not a simple function where a certain base current causes a certain resistance. It is a complicated function involving the base current, the collector current and the collector voltage. From an engineering perspective, saturation occurs when the collector current causes the collector resistor to drop the entire supply voltage. At this point, increasing the collector current would cause an increase in the voltage across the collector resistor that would violate Kirchhoff's Voltage Law. The following illustration shows why a circuit becomes saturated from an engineering point of view.

Once the collector voltage reaches zero, it is impossible for the collector voltage to go any lower. That would require a voltage across the collector resistor that is greater than the source voltage, which would violate Kirchhoff's Voltage Law.

Kirchhoff's Voltage Law Violation

From an engineering perspective, a circuit becomes saturated because it cannot violate Kirchhoff's Voltage Law. Here the collector current is 15 mA. With a 1 k collector resistor, this results in 15 volts across the resistor. This is impossible with only a 10-volt power supply.

If the base current is increased to 150 μA the collector current attempts to increase to 15 mA. This would result in 15 volts across the collector resistor. This violates Kirchhoff's Voltage Law. There is nothing to prevent the base current from exceeding 100 μA, but it is impossible for the collector current to exceed 10 mA regardless of the transistor's published h_FE.

Don't get stuck on this explanation if it seems too complicated. The saturation point is reached when the transistor's resistance bottoms out. The resistance cannot go lower, so increasing the base current cannot have a further effect on the collector current or the collector voltage.

Saturation in a real-world circuit

The circuits shown above assume the transistor has no resistance when the circuit is saturated. In the real world, the transistor will have some resistance even when the circuit is saturated. Because of this minimum resistance, and the fact that the collector current is relatively high when the circuit is saturated, the voltage across the transistor cannot fall below a certain voltage. This voltage, called V_CE(SAT), is typically around 0.4 volts. When the collector voltage reaches V_CE(SAT), the collector current has reached its practical limit. The following circuit is a practical example of a saturated circuit.

A Real-world Saturated Circuit

The collector-to-emitter voltage (VCE) cannot fall below the saturation voltage (VCE(SAT)), which is typically around 0.4 volts. This puts a corresponding limit on the collector current.

The above circuit enters saturation when the base current reaches 96 μA. This base current causes the collector current to be 9.6 mA. This collector current causes the voltage across the collector resistor to be 9.6 volts. This leaves 0.4 volts across the transistor. Increasing the base current to 100 μA, as shown above, cannot increase the collector current above 9.6 mA. Doing so would force the collector voltage below 0.4 volts, which is the saturation voltage for this transistor; its collector voltage cannot go below 0.4 volts. The collector voltage is stuck at 0.4 volts which sticks the collector current at 9.6 mA. Increasing the base current above 96 μA will not change either.

What if your circuit design requires a base current greater than 96 μA, but you can't allow the circuit to go into saturation. There are two solutions: One is to redesign the circuit to work off a higher supply voltage, the other is to decrease the value of the collector resistor.

 If the supply voltage is increased to 20 volts and nothing else is done, the base current can go as high as 196 μA. It will then take 19.6 mA of collector current to saturate the circuit; the voltage across the collector resistor can be up to 19.6 volts before Kirchhoff's Voltage Law gets in the way.

If the collector resistor is reduced to 500 ohms and nothing else is done, it will take 19.2 mA of collector current before the voltage across the collector resistor reaches 9.6 volts (19.2 mA x 500 ohms = 9.6 volts). However, decreasing the value of the collector resistor decreases the voltage gain of the amplifier.

Cut-off

Cut-off is the point where decreasing the input no longer causes a change in the output; cut-off is the opposite of saturation. Essentially, cut-off is reached when there is no base current. No base current means no collector current. No collector current means no voltage drop across the collector resistor. The output voltage then equals V_CC. The output voltage (V_C) can't go any higher; the circuit is cut-off. However, transistors leak some current. Even with no base current, there will be a tiny amount of collector current. There is some lower limit to the base current where going lower will not cause a further decrease in collector current. This causes a tiny voltage across the collector resistor keeping the collector voltage from reaching V_CC. The cut-off current is typically a few nanoamps and is labeled I_CBO (collector current with the base open) on transistor datasheets.

Common-emitter Amplifier Characteristics

The following characteristics are the characteristics of a common-emitter amplifier:

Common-collector (Emitter Follower)

The Common-collector amplifier theoretically uses the collector as the common connection. This is not obvious in most circuit diagrams. Like the common-emitter amplifier, the best way to recognize the common-collector amplifier is the location of the output, which is at the emitter (output on emitter = common-collector). This is the reason the circuit is also called an emitter follower.

The common-collector amplifier does not require a collector resistor; the collector can be connected directly to the power source. The collector current is determined by the base current multiplied by the h_FE of the transistor; an increase in base current results in a larger increase in collector current. The collector current combines with the base current and exits from the emitter, which is the output of the amplifier.

At any instant in time, the output voltage of the common-collector amplifier is approximately 0.7 volts less than the base voltage (or 0.7 volts higher using a PNP transistor). If a common-collector amplifier is amplifying an AC signal, the emitter voltage will track the base voltage with an offset of about 0.7 volts. Therefore, if the base voltage is swinging between 3 and 5 volts (2 V_P-P), the emitter voltage will be swinging between 2.3 and 4.3 volts (still 2 V_P-P). Therefore, there is no AC voltage gain with a common-collector amplifier. By definition, this is a gain of 1.

Here are the characteristics of a common-collector amplifier:

• High input impedance (requires little input current)
• Low output impedance (can deliver relatively high current)
• Output not inverted
• AC voltage gain (AV) of 1
• DC current gain of approximately hFE.

Common-Base

The Common-base amplifier uses the base as the common connection. When examining a common-base amplifier, this is usually obvious as the base is shown connected directly to ground. Another feature to help identify the common-base amplifier is that the input is at the emitter.

The following is a diagram of a simplified common-base amplifier with a list of its characteristics.

low input impedance high output impedance high voltage gain current gain less than 1
Common-base Amplifier Characteristics

The above circuits are greatly simplified. Practical transistor amplifiers have networks of biasing resistors to control the voltage and current in the circuits. Some of these biasing networks will be discussed later.

FET Amplifiers

Field effect transistor (FET) amplifiers can be made in the same configurations as the above BJT amplifiers. However, since the connections to an FET have different names, the amplifiers have corresponding different names. The FET equivalent of the common-emitter amplifier is the common-source amplifier. The FET equivalent of the common-collector amplifier is the common-drain amplifier. The FET equivalent of the common-base amplifier is the common-gate amplifier.

Review

Move your mouse over the blank lines below to see the answers.

1. What is the easiest way to identify a common-emitter amplifier?
   • The output is at the collector
2. What happens to the collector voltage (V_C) when you increase the base current?
• The voltage decreases.
3. What is the purpose of the collector resistor (R_C) in a common-emitter amplifier?
• To isolate the collector from the supply voltage—to allow the collector voltage to change when the collector current changes.
4. What is the definition of V_CC
• The supply or battery voltage—the positive voltage applied at or near the collector of the transistor.
5. What happens to the voltage gain of a common-emitter amplifier if you increase the value of the collector resistor?
• The gain increases.
6. If you place an AC signal on the base of a common-emitter amplifier, what is the phase relationship of the output?
• It is inverted, 180 degrees out of phase.
7. What is the definition of saturation?
• When increasing the input no longer affects the output.
8. If a common-emitter amplifier is saturated, what happens to the collector current when you increase the base current?
• The collector current cannot increase once the circuit is saturated.
9. What is the definition of V_CE(SAT)?
• The saturation voltage—the voltage measured from the collector to the emitter when the circuit is saturated.
10. What is the definition of cut-off?
• When decreasing the input no longer affects the output.
11. What is the definition of I_CBO?
• The cut-off current—the collector current when there is no base current (the base is open).