In many cases of applications, the output signal of IC is amplified by a transistor to drive speaker. In this article, we introduce some ways to enhance the efficiency of this kind of amplifier.
1. Audio Voltage Output
The audio output of AMFR series recording IC is voltage output. This output can be 1-ended output or 2-ended output. Some ICs with 2-ended output have internal amplifier, such as, AMFRA3009 and AMFRB6009. While some ICs with 1-ended output have no internal amplifier, such as, AMFRC0083 and AMFRC0163.
The output pin of 1-ended output is connected directly to amplifier. For the 2-ended output, any output pin can be connected to amplifier. But note that unused output pins cannot be connected to ground. Normally, the application circuit of 1-ended is as figure 1. This circuit belongs to class A amplifier since a D.C voltage is added on IC output signal. Therefore, R1 is not only a DC bias resistor, but also a current limiting resistor. The circuit in figure 1 is so simple that it can be adjusted easily.
While in some cases this circuit performs no well. It sounds in low volume. To solve this problem, we usually reduce the R1. But even reduce R1, the current will become larger, and the sound will turn into distortion. Hence, you should limit the large current by using the speakers with larger impedance. But even change speakers, the sound distortion will occur since the IC circuit is designed to match the 8Ω speaker.
With the help of oscillograph, we can see the change of Q1 collector waveform after adjusting R1 and changing speakers with different impedances. Then you can find that not only AC signals have been changed, but also DC points have been moved. When the AC signal become large, it will cause the sound distortion. Thus, the simplest circuit cannot solve problems.
Then we try another way that adding a couple to the input terminal of amplifier to increase AC signal. As shown in figure 2, it just needs to parallel connect a C1 on R1. In this way, the AC signal will be increased much. However, another problem would appear, sometimes the too large AC signal would cause the cut-off distortion. So we have to reduce the size of C1. But if the capacitor is small, the gain of amplifier for low frequency signal will also become low. It seems to be a trouble. Hence, a resistor (R2) should be series connected to C1 for adjusting the size of AC signals. Then both problems can be solved perfectly.
In the following, we’d like to analyze how to select the value of each component. As for speakers, usually the 8Ω is the common choice. But the 8Ω speaker may lead to the much larger static current. If try to set the collector voltage of Q1 as 1/2 VCC, the static current will become 1/8 of 1/2 VCC. If we use the speaker with double-size resistor, the static current will become the half of original one. So it is recommended to use speakers with 32Ω or higher impedance. However, note that if the impedance of speaker is changed, the selected resistor (R1) should be adjusted again. So choosing the impedance of speaker should be done before the selection of resistor.
As for the R1, we could select it with the help of oscillograph. We can see the collector waveforms of Q1 from oscillograph after removing AC signals. Then adjust the voltage of R1 to 1/2 VCC to get the value of R1 (see figure 3).
For selecting the C1, generally a 0.1μcapacitor is enough. But if the size of R2 is small, we could consider to use 0.47μ capacitor. Since the 0.1μcapacitor has strong impedance against to the high-frequency part of sound, it is recommended to use 0.47μ capacitor to improve low-frequency part. The selection of R2 depends on the maximum level of AC signal. To avoid the sound distortion occurring, we should adjust the collector waveform of Q1 when AC signal is in largest output (see figure 4).
2. Cout Current Output
Usually, the Cout terminal of voice IC is the same as the output of DAC in current switch mode. The current can be set in EzCode. If we set it as 3.02mA, the largest output current will be 3.02mA. The circuit of this output is shown as figure 5. In figure 5, the Cout is connected directly to the base pin of transistor. Since the output current of IC is constant, the base current is the remainder that output current minus R1 bypass current.
R1 is used for adjusting output current to match input characteristic of transistor. As the too large output current may lead to the bias points located in incorrect position or serious distortion, we could adjust the resistance of R1 to reduce the base current of transistor.
The simple way to set the resistance of R1 is to measure the current of speakers while IC is working. If the current value in speaker is equal to the half value of VCC / speaker impedance, it indicates that both voltages of speakers and transistor are 1/2 VCC, and the bias is correct.
However, this simple circuit has some problems that the volume is too low, and the current is too large. To solve these problems, you’d better to add a coupling to increase the AC signal. In this way, the volume can be increased and the current can be decreased. Please see the improved circuit of figure 6.
Similar to the way of adjusting current, adjusting output voltage can also achieve the same aim. By adjusting R3 and R1, we can ensure the bias points of transistor. Note that the resistance of R1 is mainly used to limit the maximum base current of transistor. The adjustment of R3 is used to control the bias up or down. To get the proper bias point and decrease the current, both resistors(R3 and R1) should be adjusted equally.
R2 is used for adjusting the amplitude of AC signals. It can be adjusted until the sound become the largest and no distortion occurs.
C1 is AC coupling resistor. The larger C1 can improve the low-frequency characteristic. In general, we use 0.1u capacitor. If select 0.47u, that will be better.
