____________________________________________________________________________ Amplifiers
5-42. R1 acts as a load resistor for Q1 (the first stage) and develops the output signal of
that stage. We already discussed how a capacitor reacts to AC and DC. The capacitor (C1)
"blocks " the DC of Q1's collector, but "passes" the AC output signal. R2 develops this
passed, or coupled, signal as the input signal to Q2 (the second stage), This arrangement
allows the coupling of the signal while it isolates the biasing of each stage. This solves
many of the problems associated with direct coupling.
5-43. RC coupling does have a few disadvantages. The resistors use DC power and so
the amplifier has low efficiency. The capacitor tends to limit the low-frequency response of
the amplifier and the amplifying device itself limits the high-frequency response. This is
usually not a problem for audio amplifiers. Techniques for overcoming these frequency
limitations will be covered later in this chapter.
5-44. Before moving on to the next type of coupling, the capacitor in the RC coupling
should be considered. Remember, that capacitive reactance (XC) is determined by the
following formula:
1
XC =
2 π fC
This explains why the low frequencies are limited by the capacitor. As frequency
decreases, XC increases. This causes more of the signal to be "lost " in the capacitor.
relatively high so that capacitive reactance (XC) can be kept as low as possible. So, when a
capacitor is used as a coupling element, the capacitance should be relatively high so that it
will couple the entire signal well and not reduce or distort the signal.
Impedance Coupling
5-46. Impedance coupling is very similar to RC coupling. The difference is the use of an
impedance device (a coil) to replace the load resistor of the first stage.
5-47. Figure 5-10 shows an impedance-coupling network between two stages of
amplification. L1 is the load for Q1 and develops the output signal of the first stage. Since
the DC resistance of a coil is low, the efficiency of the amplifier stage is increased. The
amount of signal developed in the output of the stage depends on the inductive reactance of
L1. Remember the formula for inductive reactance is XL = 2π fL.
5-48. The formula shows that for inductive reactance to be large, either inductance or
frequency or both must be high. Therefore, load inductors should have relatively large
amounts of inductance and are most effective at high frequencies. This explains why
impedance coupling is usually not used for audio amplifiers.
5-49. The rest of the coupling network (C1 and R1) functions just as their counterparts
(C1 and R2) in the RC-coupling network. C1 couples the signal between stages while
blocking the DC and R1 develops the input signal to the second stage (Q2).
23 June 2005
TC 9-62
5-11
Previous Page
