The Basic Circuit Configurations of Transistors

The common-emitter (CE) connection (Fig. 4.4) is the most commonly used one because it gives the largest power amplification of all.

Fig. 4.4. Connection of a transistor in a common-emitter (CE) circuit

The current gain of such a stage, is the ratio between the peak (or rms) values of output and input alternating currents, that is, the a. c. components of collector and base currents:

=I_{out m} /I_{in m} = I_Cm /I_Bm. (4.9)

Because the collector current is tens to hundreds of times as great as the base current, has a value of several tens or hundreds.

The amplifying properties of a transistor with the common-emitter connection are stated in terms of one of transistor’s principal parame­ters-the beta current gain factor, β:

β =∆ I_C /∆ I_B with U_CE held const.

As already noted, β may practically be as high as several hundreds. The stage current gain , however, is always smaller than β because bringing the load resistance R_C into circuit reduces the collector current, I_C.

The stage voltage gain, :

= U_{out m} / U _{in m} = U_RKm / U_BEm = U_CEm / U_BEm.

Therefore, ranges from tens to several hundreds.

It follows, therefore, that the stage power gain, , may range from several hundreds to tens of thousands:

= P_out / P_in = I _{out m} U_{in m} /I _{in m} U_{in m}= .

An important quantity characterizing a tran­sistor is its input resistance R_in. For a CE connection it is:

= U_{in m} / I_{in m} = U_BEm / I_Bm

and ranges from hundreds of ohms to several kilohms.

A CE transistor amplifier stage inverts the phase of the signal voltage - the phase difference between the input and output voltages is 180°.

The common-base (CB) connection. Al­though this circuit configuration (Fig. 4.5) yields a substantially lower power gain and has a still lower input resistance than the CE connection, it is used sometimes all the same because it compares favourably with the CE connection with regard to frequency and tem­perature.

Fig. 4.6. Connection of a transistor in a common-base (CB) circuit

The current gain of a CB stage is always slightly less than unity, because the collector current is always only slightly smaller than the emitter current.

The voltage gain of a CB stage is defined as:

= U_CBm / U_EBm.

It is the same in value as in the CE connection, that is, it ranges from tens to hundreds.

Because the power gain is the product of current and voltage gains, , and is approximately equal to unity, it follows that is about the same as , that is, its value ranges from tens to hundreds.

The input resistance of a CB amplifier stage is:

= U_EBm / I_Em = /(β+1).

The CB circuit does not reverse the phase of the input voltage.

The common-collector (CC) connection. In this configuration (Fig. 4.6) the collector is a true common point for the input and output circuits because the E_B and E_C sources are
always shunted by high-value capacitors and it is legitimate to visualize them as virtual short-
circuits for alternating current. A distinction of this configuration is that all of the output
voltage is fed back to the input-it is said that a very large amount of negative feedback is used.

Fig. 4.6. Connection of a transistor in common-collector (CC) circuit

The input voltage is the sum of the alternating base-to-emitter voltage, U_BE, and the output voltage:

The current gain of a CC circuit is about the same as that of the CE connection-it has a value of several tens or hundreds. To demonstrate,

= I_Em /I_Bm = (I_Cm + I_Bm)/ I_Bm = I_Cm / I_Bm + 1= +1 ≈ β+1.

In contrast, the voltage gain is very close to, but always less than, unity:

= U_{out m} / U_{in m} = U_{out m} /(U_BEm + U_{out m}) < 1.

The value of U_BEm is a few tenths of a volt, and that of U_{out m} is several volts so that U_BEm «U_{out m}. In consequence, is approximately equal to unity.

The input resistance of a CC amplifier stage is hundreds of kilohms, and this is an important advan­tage of this circuit configuration. Thus,

= U_{in m} / I_{in m} = (U_BEm + U_{out m})/ I_Bm = +(β+1)R_E .

The output resistance of a CC stage is relatively small, usually tens kilohms or even hundreds of ohms.

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