Common Base - Overview of Characteristics

Overview of Characteristics

Several example applications are described in detail below. A brief overview follows.

• The amplifier input impedance R_in looking into the emitter node is very low, given approximately by

,

where V_T is the thermal voltage and I_E is the DC emitter current.

For example, for V_T = 26 mV and I_E = 10 mA, rather typical values, R_in = 2.6 Ω. If I_E is reduced to increase R_in, there are other consequences like lower transconductance, higher output resistance and lower β that also must be considered. A practical solution to this low-input-impedance problem is to place a common-emitter stage at the input to form a cascode amplifier.

• Because the input impedance is so low, most signal sources have larger source impedance than the common-base amplifier R_in. The consequence is that the source delivers a current to the input rather than a voltage, even if it is a voltage source. (According to Norton's theorem, this current is approximately i_in = v_S / R_S). If the output signal also is a current, the amplifier is a current buffer and delivers the same current as is input. If the output is taken as a voltage, the amplifier is a transresistance amplifier, and delivers a voltage dependent on the load impedance, for example v_out = i_in R_L for a resistor load R_L much smaller in value than the amplifier output resistance R_out. That is, the voltage gain in this case (explained in more detail below) is:

.

Note for source impedances such that R_S >> r_E the output impedance approaches R_out = R_C || .

• For the special case of very low impedance sources, the common-base amplifier does work as a voltage amplifier, one of the examples discussed below. In this case (explained in more detail below), when R_S << r_E and R_L << R_out, the voltage gain becomes:

,

where g_m = I_C / V_T is the transconductance. Notice that for low source impedance, R_out = r_O || R_C.

• The inclusion of r_O in the hybrid-pi model predicts reverse transmission from the amplifiers output to its input, that is the amplifier is bilateral. One consequence of this is that the input/output impedance is affected by the load/source termination impedance, hence, for example, the output resistance, R_out, may vary over the range r_O || R_C ≤ R_out ≤ (β + 1) r_O || R_C depending on the source resistance, R_S. The amplifier can be approximated as unilateral when neglect of r_O is accurate (valid for low gains and low to moderate load resistances), simplifying the analysis. This approximation often is made in discrete designs, but may be less accurate in RF circuits, and in integrated circuit designs where active loads normally are used.