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All amplifiers have three fundamental properties, gain, input impedance, and output impedance. These properties can be represented using a general amplifier model like the one in Figure 8-1.  Figure 8-1. General amplifier model. The diamond shape in the amplifier model represents the gain of the circuit. Gain is a multiplier that exists between the input and output of a circuit. For example, a gain of 50 indicates that the output signal is fifty times as great as the input signal. There are three types of gain: voltage gain ( Technically, gain is a ratio of an output value to its corresponding input value. This definition is illustrated by the following relationships:    Since gain is a ratio of like values, it has no unit of measure. This point is demonstrated in Example 8.1 of the text. The above equations are misleading, because they imply that the gain of a circuit is determined by its input and output values. The gain of a circuit is actually determined by its component values. When the gain of a circuit has been calculated, it can be used to determine the output from the circuit for a specified input. This point is demonstrated in Example 8.2. A voltage amplifier is a circuit that is designed to provide a specific value of voltage gain. The general model for a voltage amplifier is shown in Figure 8-2. Note that the diamond has been modified to represent the product of the circuit voltage gain and the input signal voltage (  Figure 8-2. Voltage amplifier model. When a signal source and load are connected to the voltage amplifier model, we get the circuit shown in Figure 8-3. Note that:
Also note the The input and output voltage dividers affect the values of source and load voltage as follows:   The voltage-divider equations indicate that  Figure 8-3. Voltage amplifier with its source and load. The effective voltage gain of a circuit is the ratio of its load voltage to its source voltage. Since The ideal voltage amplifier, if it could be constructed, would have the following characteristics (among others):
With values of
BJT Amplifier Configurations There are three BJT amplifier configurations, each with its unique input/output characteristics. These amplifier configurations are shown in Figure 8.12 of the text. The characteristics of the three configurations can be summarized as shown in Table 8-1. TABLE 8-1
*Also referred to as an emitter-follower.
To identify the configuration of any BJT amplifier, you simply identify the signal input and output terminals. The third terminal is the common terminal. This can be seen by comparing amplifiers listed in the table with their input and output terminals.
Amplifier Classifications Amplifiers are classified according to their ability to deliver power to a load. The four primary classifications for BJT amplifiers are listed (along with their characteristics) in Table 8-2. Amplification is accomplished by transferring power from the amplifier’s dc power supply to the input signal. The ideal amplifier would deliver 100% of the power taken from its dc power supply to its input signal. This cannot happen in practice (at this point) because the components in the amplifier each dissipate some measurable amount of power. (See Figure 8.18 of the text.) The efficiency of an amplifier is the percentage of the power drawn from the dc power supply that is actually delivered to the load. As shown in Table 8-2, each amplifier classification has a maximum theoretical efficiency rating. TABLE 8-2
*Maximum theoretical value. Actually efficiency values are significantly lower.
Decibels Component and system spec sheets often list gain values in decibel form. A decibel (dB) is a ratio of output power to input power, equal to 10 times the common logarithm of that ratio. The dB power gain of an amplifier is found using
where One benefit of representing gain in dB form is that very large (and small) values can be represented using few digits. (For example, a power gain of 1,000,000 is equal to 60 dB.) However, the drawback to using dB values is that they cannot be used in standard circuit calculations without first being converted back into standard numeric form. A dB power gain is converted to standard numeric form using
Once converted to standard numeric form, Another advantage of expressing gain in dB form is the fact that positive and negative dB values represent reciprocal gains and losses. For example, 3 dB represents a power gain of approximately 2, while –
3 dB represents a power loss of
The final advantage of expressing gain in dB form is the fact that multiple gains (and losses) are found by simply adding the individual dB values. For example, Figure 8-4 shows the block diagram for a two-stage amplifier. Note that the total dB power gain of the circuit equals the sum of the individual dB power gains.  Figure 8-4. dB gain values are additive.
On some spec sheets, power values are listed in dBm form. A dBm value is a power value, referenced to 1 mW. Power, in dBm, is found using
Unlike standard dB values (which represent power ratios), a dBm value represents an actual power level. This concept is demonstrated in Examples 8.11 and 8.12 of the text. Like power gain, voltage gain is often represented in dB form. The dB voltage gain of an amplifier is found using
where When the dB voltage gain of a circuit changes, the dB power gain changes by the same factor (assuming that all other values remain constant). This relationship is expressed as
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), current gain (
), and power gain (
). 
).
) forms a voltage divider with the source resistance (
)
) forms a voltage divider with the load resistance (
)
and 
are typically resistive in nature, so their values can be combined algebraically with 
and 
.
and 
. These relationships are demonstrated further in Examples 8.3 and 8.4 of the text.
and 
, the effective voltage gain of a voltage amplifier is lower than the calculated voltage gain of the amplifier itself.
and 
, the input and output voltage dividers would be effectively eliminated, leaving 
and 
. As a result, the calculated and effective voltage gains of the circuit would be equal.
. Example 8.7 of the text demonstrates the calculation of dB power gain.
can be used (along with 
) to calculate circuit output power, as demonstrated in Example 8.9 of the text.
. This relationship can be stated mathematically as follows:
.
. The dB voltage gain of an amplifier is calculated as demonstrated in Example 8.13 of the text.