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The Frequency Behaviour of Transistors

Diffusion Currents in Semiconductors | The Capacitances of a Semiconductor Diode | Semiconductor Diodes as Rectifiers | Structures of Semiconductor Diodes | The Tunnel and Inversed Diodes | Microwave Semiconductor Diodes | Physical Processes in a Transistor | The Basic Circuit Configurations of Transistors | Models of Transistors | Bias Supply and Temperature Compensation for Transistors |


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A rise in frequency leads to a fall in the amplification supplied by a transistor. Two factors may be held responsible for this oc­currence. Firstly, the collector junction capa­citance CjC produces a detrimental effect at high frequencies. At low frequencies, CjC presents a very high reactance; also rC is very large (as a rule, rC is many times RL), so we may take it that all of the current aIE is flowing to the load resistor or that K1 ≈ a. At some high frequency, however, the CjC reactance drops to a relatively low value and a sizeable proportion of the current supplied by the generator divides into this reactance and a smaller current flows through RL. In consequence, KI, KU, KP and output power suffer a reduction.

The emitter diffusive capacitance CdE likewise falls off with a rise in frequency, but it is always shunted by the low emitter-junction resistance rE, and so its detrimental effect may be felt only at very high frequencies when l/ωCE is compa­rable in magnitude with rE.

Fig. 4.19. Equivalent circuit of a transistor, with its junction

 

The CE circuit has a poorer frequency performance in comparison with the CB con­nection.

It is customary to set the allowable limit of fall in the two current gains by 30% from their values α0 and β0 at low frequencies (usually, a frequency of 1 kHz is taken). The frequencies at which the gain falls so that and , are called the common-base current-gain cutoff frequency and the common-emitter current-gain cutoff frequency, respectively for CB and CE circuits. In this text, they will be labelled as fα and fβ.

Because the beta current gain decreases at a far higher rate than the alpha current gain, fβ is markedly lower than fα. We may write:

fβ = fα /(β+1).

Figure 4.20 shows an approximate plot illu­strating how the alpha and beta current gains fall off with rising frequency.

Fig. 4.20. Reduction in the alpha and beta gains with rising frequency

 

The design equations include also what is known as the gain-bandwidth product. It is designated as fT and defined as the frequency at which the common-emitter forward current gain | KI |is equal to unity, that is, when a CE circuit ceases to amplify current.

 

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