Utilise audio amps for voltage splitting (Part 3)

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1 Utilise audio amps for voltage splitting (Part 3) Here's a look at voltage splitters based on power audio amplifiers. By Petre Petrov Electronics Engineer Part 1 gave an overview of voltage splitters and their various implementations. Part 2 tackled some features of audio power amplifiers. Voltage splitters based on power audio amplifiers From the point of view of the power supply the audio amplifiers are generally of two types. The first type is used mainly in applications with a single power supply. Some can be used with a dual power supply but that is an unusual application and sometimes it is not possible. Examples of these types of PAAs are LM386, LM380, LM384, TBA820M and similar. The second type of PAAs are used mainly in applications with a dual power supply, but some simple additional circuit can work also with a single power supply. Examples of these types of PAAs are TDA2030, TDA2040, TDA2050, LM1875 and similar ICs. Although there are a lot of similarities between these PAAs we should note a lot of differences and particularities between them. Figure 6 presents the circuit diagram of a very simple non-adjustable voltage splitter using an LM386. Figure 6: Non adjustable voltage or adjustable splitter with LM386. The circuit works in the full power supply range of the IC, but the output current is limited by the power dissipation of the IC, which depends on the power supply. Resistors R2 and R3 are not obligatory but have a protection role (e.g., if we omit them we should take special consideration of the pcb layout because the inputs of LM386 can capture a lot of noise and that noise will be amplified and will be seen at the output of the IC). The input resistance of the LM386 is around 50k and the inputs are with PNP transistors and can go to ground. Resistor R4 may be necessary for the stability of the IC. The trimmer potentiometer P1 is used to adjust the output voltage. We can apply the offset from P1 to the inverting or non-inverting input of the IC. Jumper J1 is for application of the offset to the non-inverting input and J2 is for the application of the offset to the inverting input. J1, J2, R1 and P1 are not obligatory and can be omitted in the simplest case. Sometimes we are in need of more current, higher voltages and higher power dissipation than those offered by the LM386. Also we may need to adjust the output voltages from the VS. In some of these cases we may use more EE Times-Asia eetasia.com Copyright 2014 emedia Asia Ltd. Page 1 of 7

2 powerful ICs such as the LM380 and LM384. These two ICs have practically the same internal circuit, but the LM384 can provide more power. Also, the LM380 is produced in 8- and 14-pin packages while the LM384 is produced only in a 14-pin package. Both ICs can require additional heat sinks. Figure 7 presents the circuit which can be used as an adjustable or non-adjustable VS. Figure 7: Adjustable or non-adjustable voltage splitter with LM380 in a DIP8 and with LM380 or LM384 in a 14-pin DIP. The first half of the circuit is a VS with a LM380 in an 8-pin package; the second half of the circuit is a VS with an LM380 or LM384 in a 14-pin package. Both parts of the circuit are independent. The jumpers J1 to J4 can be used for adjustment of the output voltages. In all cases the input voltage of IC1 will be amplified around 50 times and will appear between the output of the IC and GND1. That gain should be taken into consideration if we wish to adjust the output voltage with a trimmer potentiometer. In some cases we may wish to have a stable output voltage between the output of the VS and the input ground terminal. In these cases we should apply a reference voltage at the appropriate input of the PAA. Figure 8 presents simple adjustable voltage splitter with an LM386. Figure 8: An adjustable voltage splitter with an LM386 and with a reference input voltage from an LM The reference input voltage of around 1.2V is obtained from an LM In that case the internal gain of the LM386 is 20; the minimal gain of that IC is around nine. The adjustment of the output voltage is made with the trimmer potentiometer P1. The jumpers J1 and J2 are used to apply the reference voltage to the inverting or noninverting input of the PAA. The resistors R3, R4 and R5 are not obligatory, but improve the parameters of the VS. EE Times-Asia eetasia.com Copyright 2014 emedia Asia Ltd. Page 2 of 7

3 The IC TBA820M also provides several options to create adjustable and non-adjustable VSs. That IC has the possibility to externally adjust the voltage gain and the frequency compensation. Figure 9 presents the circuit of a simple VS around the TBA820M. Figure 9: A VS circuit using the TBA820M. The input pin 3 of the IC can go to the ground. That input is protected with an internal resistor of around 2k. The AC gain of the IC should be set to the minimum in order to reduce the noise at the output of the IC. The frequency compensation is adjusted with C4. If needed, the output voltage is adjusted with the trimmer potentiometer P1. Only one of the jumpers J1 and J2 should be closed, depending on the required mode of operation. If we are in need of more current and power in single polarity applications we can use a VS based on single power audio amplifiers such as the TDA2002 (10W) and TDA2003 (10W) and dual PAAs such as the TDA2004R (6W+6W), TDA2005R/M/S (11W+11W), TDA2009/A (12W+12W) and similar. The DC test circuit of these ICs can be used as simple DC non-adjustable VSs. Here we will examine one of these cases. Figure 10 presents the circuit of a VS based around a TDA2003. The circuit also works with the older and less popular now TDA2002. Figure 10: A VS with the TDA2003 or older and less popular now TDA2002. For a power supply of 14.4V the output voltage of the IC is between 6.1V and 7.7V. The inputs of the IC are connected to single internal NPN transistor. The negative feedback between the output of the circuit and the emitter of the input transistor is accomplished by R2 and R3. The voltage gain of the IC should be set to minimum in order to have minimum output noise. The output voltage is adjusted with the trimmer potentiometer P1. PAAs such as the TDA2030/40/50, LM1875, LM1876 (dual PAA), LM1877 (dual PAA) can be considered as some sort of power operational amplifiers with differential inputs, but nevertheless there are some significant differences among these PAAs. E.g., the input stage of the TDA2030/40/50 uses input PNP transistors while the input stage of the LM1876 and LM1877 is with NPN transistors. Care should be taken about the maximum differential input voltages of the ICs and if needed we should add external protection with resistors, diodes and capacitors. For example, the maximum input voltage of some ICs can be very low, e.g., only 0.7V. EE Times-Asia eetasia.com Copyright 2014 emedia Asia Ltd. Page 3 of 7

Figure 11 presents the circuit of a VS using a TDA2030, TDA2040 or TDA2050. Figure 11: A VS with a TDA2030, TDA2040 or TDA2050. The IC works as a DC amplifier with gain set by R3 and R2.

4 Figure 11 presents the circuit of a VS using a TDA2030, TDA2040 or TDA2050. Figure 11: A VS with a TDA2030, TDA2040 or TDA2050. The IC works as a DC amplifier with gain set by R3 and R2. The gain should be above the minimal value and appropriate external frequency compensation should be used. The problem with the DC configuration is that the IC amplifies its DC offset. The internal offset depends on the temperature. In most cases that is not important but in all cases it should be taken into consideration. In figure 11 the output voltages are adjusted in large scale with the trimmer potentiometer P1 and that is one of the advantages of that VS. The gain of the IC is calculated with the formula below: Av = 1 + R3/R2. In order to improve the stability of that and similar VSs we may need to put a small high-current inductor in parallel with the resistor in series with the output (in this case the resistor is R4*). Sometimes we are in need of two or more DC power supplies that use the ground line of the input voltage. In these cases we can use several single amplifiers or dual, triple or quadruple PAAs or OAs. Figure 12 shows the circuit diagram of an adjustable power supply that has three output power supplies - V1, V2 and V3 - that share a common ground GND. Figure 12: Adjustable power supply which has three output power supplies V1, V2 and V3 which shares a common ground GND. EE Times-Asia eetasia.com Copyright 2014 emedia Asia Ltd. Page 4 of 7

5 In fact this is a VS from the second type (figure 1b). That circuit is built around the dual low-power audio amplifier TDA2822M. The TDA2822M features PNP input transistors and an internally fixed AC gain of around 100 (36 to 41 db), but here the IC is used as a dual DC amplifier. At a power supply of 6V the quiescent output voltage at each output is typically 2.7V. We may adjust the output voltages V2 and V3 with the trimmer potentiometers P1 and P2. The Jumpers J1 to J4 select the mode of operation of the VS. Voltage splitters based on audio operational amplifiers Many modern electronic systems require power supply currents well below 50mA. And we are in need of shortcircuit protection of these voltages. Audio operational amplifiers (AOAs) can be a good choice for VSs in that case. Modern industry offers a large variety of single, dual and quadruple OAs applicable in these VSs. Table 2 gives some operational amplifiers appropriate for VSs. Table 2: Some high current, high-voltage or high-power operational amplifiers appropriate for voltage splitters. * - The gain is set externally in wide range. ** - approximate value, please see the data sheet. Figure 13 presents the circuit diagram of a VS based around dual paralleled OAs. Figure 13: A VS based around dual paralleled op amps. We may use practically any dual op amp with a current limiting function that is stable at unity gain and stable with a high capacitive load. The IC is selected according to the required output voltages, current and power dissipation. The output voltage is adjusted with the trimmer potentiometer P1. The equalisation resistors R3 and R4 are obligatory and depend mainly on the stability of the OAs when driving capacitive loads. If the current provided by two paralleled OAs is not enough, we may use three or four paralleled OAs under the condition that the power dissipation of the package of the IC is not exceeded. Figure 14 gives the circuit diagram of a VS based around quadruple paralleled OAs. EE Times-Asia eetasia.com Copyright 2014 emedia Asia Ltd. Page 5 of 7

6 Figure 14: A VS based around quadruple paralleled OAs. The dual switch S1A+S1B is for selecting two options: In position 1 we have two independent voltage splitters and the output voltages are adjusted with P1 and P2. In position 2 we have a single voltage splitter with four paralleled OAs and the output voltage is adjusted with P1. In the VS we may use a lot of ICs starting with low-cost OAs like the LM324 and TL084 or using the more expensive OPA4132, OPA4134, etc. Figure 15: A VS based around an L165 power operational amplifier. Voltage splitters based on DC amplifiers and motor drivers Modern industry offers ICs specially designed to work as power operational amplifiers, e.g., the L165. The L165 is EE Times-Asia eetasia.com Copyright 2014 emedia Asia Ltd. Page 6 of 7

7 similar to the TDA2030, TDA2040 and TDA2050, and to some degree the LM1875. It is a special type of operational amplifier designed to work in power supplies and servo applications compensated for gain above 10 (20dB). Figure 15 gives the circuit diagram of a VS based around an L165. The output voltage is adjusted with the trimmer potentiometer P1. The input of the IC is protected with C1, R3, D1 and D2. The gain of the IC is adjusted with R5 and R4 and should be above 10. Voltage splitter with amplifiers and voltage regulators We may use PAA and voltage regulators in the same VS and obtain additional regulated output voltages around the virtual ground of the output. Figure 16 gives the simplified circuit diagram of a VS built around PAA IC3. Figure 16: Simplified circuit diagram of a VS built around a PAA, an LM317 and an LM337. The output voltages V1 and -V2 depend on the PAA. The additional output voltages V3 and -V4 are produced by an LM317T and LM337T. Output voltage V3 is adjusted with P1 and -V4 is adjusted with P2. The circuit around IC3 is not given with all details. Voltage splitters based on power and high-voltage operational amplifiers Modern industry offers OAs with power supply voltages above 22V and with guaranteed output current above 15mA. Also there are OAs with significant power dissipation capability. These OAs are sometimes called highvoltage OAs (HVOAs), high-current OAs (HCOAs) or high-power OAs (HPOAs). Some of these OAs are appropriate for VSs and we should mention them because they extend the field of the applications of the VSs. Table 2 gives some examples from the series OPAxxx produced by Burr-Brown/Texas Instruments. The circuits with classical OAs and PAAs with differential inputs mentioned above in the article are applicable to these OAs. In that case, a slight redesign of the presented circuit of the VS is needed and we will not discuss them again. We will mention that many of theses HVOAs and HPOAs can be used in voltage splitters from 36V and 48V batteries and DC power supplies. About the author Petre Petrov has worked as a researcher and assistant professor in the Technical University of Sofia (Sofia, Bulgaria) and as a lecturer and expert lecturer in Morocco. Now he is working as electronics engineer in Bulgaria. EE Times-Asia eetasia.com Copyright 2014 emedia Asia Ltd. Page 7 of 7

Chapter 9: Operational Amplifiers

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