Communication electronic circuits

Author: Xie Yuanqing / Edited by: Sun Weiling
Publisher:
Publish Date: 2000-02-01
Features:
Fragment: The output stage of a bandpass amplifier can be divided into two categories based on the significant differences in its load. One category is characterized by a low-impedance load, such as coaxial cables, with standard impedances of 75Ω or 50Ω. Broadband amplifiers in communication equipment and instruments fall into this category. The other category is characterized by a high-impedance load, such as the output stage of an oscilloscope deflection amplifier or a video amplifier with a screen-grid cathode as its load. For low-impedance output stages, there is generally no high demand for voltage dynamic range, but the requirement for current dynamic range can be quite substantial. For example, to produce a 1V RMS sine wave voltage across a 50Ω resistor, the peak-to-peak current reaches 56mA. If an impedance matching network is inserted between the output stage and the load cable, the requirement for the device's current dynamic range may sometimes almost double. For high-impedance output stages, such as the output stage of an oscilloscope deflection plate amplifier, the load is the oscilloscope's deflection plates, which are essentially capacitive. When discussing the output stage of such amplifiers, it is necessary to consider the upper cutoff frequency. Since a higher upper cutoff frequency requires a smaller load resistance under the same load capacitance conditions, a smaller load resistance leads to a larger required current dynamic range for the same output voltage. Because people are accustomed to considering issues from the perspective of voltage, designers of circuits for low-impedance loads may mistakenly assume that since the output voltage requirement is not high, any device can be selected. As a result, when determining the quiescent operating point current, they may think that a few milliamperes are sufficient. The outcome is that the device enters the cutoff region due to the low-impedance load, and it fails to deliver the required voltage swing, even with a relatively small voltage amplitude. For high-impedance load output stages, circuit designers may simply consider the device's voltage rating when selecting components without considering the current capacity. The result is either an upper cutoff frequency that does not meet the requirements or, if the upper cutoff frequency is taken into account, insufficient current swing due to a very small load resistance, making it impossible to deliver the required output voltage. Below, we analyze the working characteristics of broadband amplifier output stages.
1. Low-Impedance Load Output Stage
1. "Negative Feedback Pair" Circuit
Figure 1-9 shows a parallel-type "negative feedback pair" circuit. In the figure, R2, R3, and C3 are impedance matching networks, C2 is a DC-blocking capacitor, and RF is the feedback resistor. The impedance represented by the dashed line in the figure represents the input impedance of the cable (equal to the characteristic impedance ZC). The output impedance seen from the emitter of VT2 is generally much lower than the cable's characteristic impedance. According to the impedance matching circuit discussed earlier, R3 should equal ZC, and R2 should be slightly smaller than ZC. As a result, the current I3 and the load current IL are of the same order of magnitude, while the current I2 is less than twice IC. Meanwhile, the output voltage at VT2's emitter will be more than twice U. The AC current Ie at VT2's emitter equals I2, the vector sum of I1 and If. Generally, RF is much larger than RE2, and the main current that shunts I2 is I1. The quiescent operating point current IEQ2 of VT2 should be larger than the AC component Ie to avoid nonlinear distortion. Since the AC load is smaller than the DC load, the nonlinear distortion that occurs is cutoff distortion. Once the load and matching circuit parameters are determined, it is not difficult to calculate the required quiescent operating point current IEQ2 based on circuit analysis knowledge.
2. Operational Amplifier and Low-Impedance Load Connection
Most operational amplifiers have an output current limit of less than 10mA. When connected to a low-impedance cable, it is difficult to obtain a sufficiently large output voltage. For example, a 10mA current can only produce 0.5V of voltage across a 50Ω resistor. To address this, an emitter follower can be added to the output of the operational amplifier. Due to the deep negative feedback in the circuit, the equivalent output resistance at the emitter of VT is very low. To achieve impedance matching at the input, a resistor R equal to the cable's characteristic impedance is connected in series between the emitter of VT and the cable interface. When the operational amplifier is powered by dual supplies, the circuit can achieve zero input and zero output, thus eliminating the need for a DC-blocking capacitor at the output. The lower limit of RE is determined by the allowable value of the output current shunt.
The circuit shown in Figure 1-10 has very deep negative feedback. Due to the low load value, the dynamic margin of the output transistor VT changes in different directions when the current increases and decreases. The dynamic margin is larger when the current increases. Therefore, when the working signal is large, the effectiveness of negative feedback differs in the two directions. Figure 1-11 shows the waveform of the output pulse u. under the action of a large-amplitude rectangular input pulse ui. The rising edge is steeper, while the falling edge is slower, because when the current increases, VT has a larger dynamic range margin.