The SHUNT-DIODE DETECTOR (figure 3-6)
is similar to the series-diode detector except that the output variations are current pulses rather than voltage pulses. Passing this current through a shunt resistor develops the voltage output. The input is an rf modulated envelope. On the negative half cycles of the rf, diode CR1 is forward biased and shunts the signal to ground. On the positive half cycles, current flows from the output through L1 to the input. A field is built up around L1 that tends to keep the current flowing. This action integrates the rf current pulses and causes the output to follow the modulation envelope (intelligence) closely. (Integration was discussed in NEETS, Module 9, Introduction to Wave-Generation and Wave-Shaping Circuits.) Shunt resistor R1 develops the output voltage from this current flow. Although the shunt detector operates on the principle of current flow, it is the output voltage across the shunt resistor that is used to reproduce the original modulation signal. The shunt-diode detector is easily identified by noting that the detector diode is in parallel with both the input and load impedance. The waveforms associated with this detector are identical to those shown in views (B), (C), and (D) of figure 3-5.
Figure 3-6. - Shunt-diode detector.
The series-diode detector is normally used where large input signals are supplied and a linear output is required. The shunt-diode detector is used where the voltage variations are too small to produce a full output from audio amplifier stages. Additional current amplifiers are required to bring the output to a usable level. Other methods of detection and amplification have been developed which will detect low-level signals. The next sections will discuss two of these circuits, the common-emitter and common-base detectors.
The COMMON-EMITTER DETECTOR is often used in receivers to supply an amplified detected output. The schematic for a typical transistor common-emitter detector is shown in figure 3-7. Input transformer T1 has a tuned primary that acts as a frequency-selective device. L2 inductively couples the input modulation envelope to the base of transistor Q1. Resistors R1 and R2 are fixed-bias voltage dividers that set the bias levels for Q1. Resistor R1 is bypassed by C2 to eliminate rf. This RC combination also acts as the load for the diode detector (emitter-base junction of Q1). The detected audio is in series with the biasing voltage and controls collector current. The output is developed across R4 which is also bypassed to remove rf by C4. R3 is a temperature stabilization resistor and C3 bypasses it for both rf and af.
Figure 3-7. - Common-emitter detector.
Q1 is biased for slight conduction with no input signal applied. When an input signal appears on the base of Q1, it is rectified by the emitter-base junction (operating as a diode) and is developed across R1 as a dc bias voltage with a varying af component. This voltage controls bias and collector current for Q1. The output is developed by collector current flow through R4. Any rf ripple in the output is bypassed across the collector load resistor by capacitor C4. The af variations are not bypassed. After the modulation envelope is detected in the base circuit, it is amplified in the output circuit to provide suitable af output. The output of this circuit is higher than is possible with a simple detector. Because of the amplification in this circuit, weaker signals can be detected than with a simple detector. A higher, more usable output is thus developed.
Q.15 Which junction of the transistor in the common-emitter detector detects the
Another amplifying detector that is used in portable receivers is the COMMON-BASE DETECTOR. In this circuit detection occurs in the emitter-base junction and amplification occurs at the output of the collector junction. The output developed is the equivalent of a diode detector which is followed by a stage of audio amplification, but with more distortion. Figure 3-8 is a schematic of a typical common-base detector. Transformer T1 is tuned by capacitor C3 to the frequency of the incoming modulated envelope. Resistor R1 and capacitor C1 form a self-biasing network which sets the dc operating point of the emitter junction. The af output is taken from the collector circuit through audio transformer T2. The primary of T2 forms the detector output load and is bypassed for rf by capacitor C2.
Figure 3-8. - Common-base detector.
The input signal is coupled through T1. When capacitor C3 is tuned to the proper frequency, the signal is passed to the emitter of Q1. When no input signal is present, bias is determined by resistor R1. When the input signal becomes positive, current flows through the emitter-base junction causing it to be forward biased. C1 and R1 establish the dc operating point by acting as a filter network. This action provides a varying dc voltage that follows the peaks of the rf modulated envelope. This action is identical to the diode detector with the emitter-base junction doing the detecting. The varying dc voltage on the emitter changes the bias on Q1 and causes collector current to vary in accordance with the detected voltage. Transformer T2 couples these af current changes to the output. Thus, Q1 detects the AM wave and then provides amplification for the detected waveform.
The four AM detectors just discussed are not the only types that you will encounter. However, they are representative of most AM detectors and the same characteristics will be found in all AM detectors. Now let's study some ways of demodulating frequency-modulated (fm) signals.
Q.18 Which junction acts as the detector in a common-base detector?