Without the bypass capacitor, the emitter is no longer at AC ground. With the bypass capacitor, the gain of a given amplifier is maximum and equal to R C/ r' e. Voltage Gain Without the Bypass Capacitor The term common refers to one of the three terminals (E, B, or C) that is referenced to AC ground for both input and output signals. Each of the parameters carries a second subscript to designate the common-emitter (e), common-base (b), or common-collector (c) amplifier configuration.įigure 8: Subscripts of h parameters for each amplifier configuration The four basic AC h parameters are h i, h r, h f and h o. H Parameters Figure 7: Basic ac h parameters The AC emitter resistance, r e' is the most important of the r parameters for amplifier analysis.įor the approximate value of r e', you can useĬomparison of the AC Beta (β ac) to the DC Beta (β DC) Figure 6: Difference between βac and βDC These are shown with a transistor symbol in Fig. The collector acts as a dependent current source of α acI e or, equivalently, β acI b(diamond-shaped symbol). This is the resistance “seen” looking into the emitter for a forward-biased transistor. Transistor AC operation: A resistance appears between the emitter and base terminals. 4(b).įigure 5: Relation of transistor symbol to r-parameter model The resulting simplified r-parameter equivalent circuit is shown in Fig. The AC collector resistance r c' is usually several hundred kilohms and can be replaced by an open. The effect of the AC base resistance r b' is usually small enough to neglect, so it can be replaced by a short. R Parameter Transistor Model Figure 4: r-parameter transistor model R Parameters Figure 3: Commonly used r parameters for BJTs A transistor always produces a phase inversion between the base voltage and the collector voltage.
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