Simple PS voltage splitters based on audio amplifiers – Part 2
Notes about some features of audio power amplifiers
Table 1 gives some of the power audio amplifiers (PAAs) that are appropriate for voltage splitters (VSs). We should make short notes about some features of the PAAs and op amps (OAs) used in VSs. That is important in order to understand better the circuits proposed below. In all cases the datasheets and application notes of the involved ICs should be considered carefully.
* – the gain can be or is adjusted with external components
(sometimes wider range that specified in the table is possible).
** – Please see the datasheets and the application notes (in all cases).
The input stage of the PAAs and OAs
The input stage and the parameters associated with that stage are very important to obtain stable and reliable operation of the VSs. Generally speaking there are three types of input stages of the PAAs and OAs:
- differential amplifiers based on PNP transistors,
- differential amplifiers based on NPN transistors,
- input stage based on single transistor (usually NPN) which is comparing the input signal and the signal from the negative feedback from the output.
Each of the inputs mentioned above has some advantages and disadvantages and particularities. For example with the differential amplifiers based on PNP transistors we can go typically to the input ground, which is usually not possible for the second and third type of input stages.
The input stage and the parameters associated with that stage are very important to obtain stable operation of the VS. Some of the inputs of the ICs have effective built-in protection but others are not well protected and should be taken into consideration.
Power supply range
The power supply range of the PAAs and OAs can have a lot of particularities. For example, some of the ICs are specially designed for a single power supply, e.g., LM380, LM386, LM324, etc. Many of these ICs automatically keep the DC output voltage around half of the applied power supply. Care should be taken about the DC levels of the input pins of these ICs.
If these circuits are used with a dual power supply, we should take some precautions in order not to damage them, e.g., the load may have a particular configuration. Other PAAs as TDA2030/40/50 and others can work with single and dual, symmetrical and non-symmetrical power supplies.
The power dissipation of the PAA in the VS should be evaluated in every particular case. That parameter depends mainly on the type, size and symmetry of the loads driven by the VS.
In the case of symmetrical loads, the average power dissipation from the VS can be very low. In the case of heavy unsymmetrical loads, the average power dissipation from the VS can be high and close to the power dissipation of the DC amplifier. Most of the PAAs have internal thermal protection but that is not the case for all amplifiers.
Switch ON and OFF particularities
Some of the PAA can have features as mute control, standby mode, etc. These features can be used and the VSs based on these amplifiers can have switch ON and OFF, power reduction modes, standby mode, etc. If we use these special modes we should take into considerations the transitional processes during the activation and deactivation on the modes.
Offset of the output voltage
Ideally, in single polarity operations the PAA – e.g., LM386, LM380, LM384, etc. – should divide the power supply in exactly two equal parts. In practice we may find a significant difference between both output voltages, e.g., more than 10%. In some cases we are in need of additional external components to adjust the output voltages to the required voltage levels and stability.
The DC output offset of the simple PAA can be significant, e.g., with a power supply of 18V the amplifier LM380 can have an output voltage of 9V ±1V, or the deviation of the output voltage from the ideal middle 18V/2 = 9V is around ±1V without external adjustment.
Sometimes the minimum and maximum quiescent output voltages are not given in the data sheet, but in most cases the maximum tolerance is below ±15% from the ideal voltage. From that point of view it is most profitable to use an IC similar to the classic OA as a VS and to apply the corresponding techniques for adjusting the output voltages and to reduce the offsets and the temperature dependence of these offsets. The PAAs usually do not have pins for external offset correction and we should use external components for that purpose.
Maximum output current
The maximum working output current of the PAA is not always specified, e.g., LM386. For some PAAs we have the maximum peak current or the maximum short circuit current, e.g., LM380, LM384 etc. The working values of that current can be derived from the maximum output power and the load resistance, which are always given in the data sheet.
We should differentiate between the maximum working current, the absolute output current and the short circuit current. These currents are different and can have different definitions and relations in the different ICs. Most PAAs have effective over-current or short-current protection. It is preferable to use PAAs with up to half of the maximum output current if it is specified.
The maximum output current of the IC in the VS is important. It may not correspond to the required current from the VS. If that current is too low we should add external current buffers, e.g., we can add transistors to the PAAs or the OAs. But if that current is too high we should add external current limiters and usually these limiters are in the powers supply line of the IC, e.g., we may use LM317M/T and LM337M/T to limit the output current of TDA2050 up to around 1.5A and to use the full power dissipation of the PAA.
Input/output protection of the IC
The inputs and outputs of some of the audio circuits are protected internally from voltages and current outside the safe limits. The internal protection components are usually given on the circuit diagrams published by the manufacturers. But that is not the case for all ICs.
For example, the internal protection components are not always explicitly given, or simply do not exist. In all cases we should consider the use of external protection. Usually this is implemented with additional diodes and resistors and sometimes capacitors as shown on some of the figures. Some amplifiers should be used with protective diodes in parallel with the output transistors.
Input current and offset currents
The input and offset currents of many of the PAAs are relatively low, e.g., below 1µA. Usually they are not a problem in the discussed applications.
Minimum and maximum voltage gain Many PAAs and some AOAs have minimal AC gain. If used below that gain the ICs can become unstable and oscillate. The frequency of the oscillation can be high (e.g., 0.5MHz to 10MHz) or low (a few Hertz) depending on the circuit and its implementation or the pcb.
Also it is important to note the difference between the AC and the DC gain of the amplifier. In both cases the requirements for the minimum gain can be different and should be respected in order not to have unstable operations.
Many PAAs and AOs have the possibility of connecting external passive components to provide appropriate frequency compensation for the selected gain, e.g., TBA820M. For other ICs we should use external frequency compensation (FCs).
PAAs similar to the operational amplifiers
Many audio power amplifiers such as TDA2030, TDA2030, TDA2050 and LM1875 are, roughly speaking, powerful operational amplifiers. They have differential inputs and a single output. We can use external components to obtain practically any AC and DC gain and any frequency compensation (FC).
The problem is that when we add simple external FC we reduce the slew rate and the bandwidth of the amplifier. As a result, we increase the time of reaction of the VS to the change of the loads. Usually that is not a problem and can be compensated with appropriate capacitors in parallel with the voltages produced by the VS, but the IC should be stable with that capacitive load.
Adding external transistors to the PAA
Transistors are low priced and widely used electronic components. When in need of higher output current and power dissipation from the VS it is preferable to add external transistors to the PAA. That makes the VS more resistant to negative events from the loads. Also the VS can provide larger output current and power dissipation. The transistors can have significant power dissipation.
Usually the additional transistors work with collector-to-emitter voltages above 3V and can have significant gain in the VS. Also the THD and IMD of the VS are not important here and the quiescent current of these transistors can be very low. The voltage drops over the output transistors of the IC and over the additional transistors is usually significant and do not limit the gain of the ICs.
AC and DC gain
Normally the PAA are used as AC amplifiers in the audio range. But many of them can also be used as DC amplifiers. One of the limiting factors in that application is the power dissipation. Also in all applications we should take care about the minimum gain when the PAA is stable and the external FC to keep the amplifier stable.
Quality of the external capacitors
The external capacitors are not part of the IC but they interact with the internal components of the IC and should be mentioned briefly here. Care should be taken about the quality of the capacitors connected to the output of the ICs, between the pins for the power supplies of the ICs, and in the feedback and the frequency compensation circuits.
Some capacitors can prevent oscillation of the ICs, the others can provoke oscillation always or under some conditions. That is due to the parasitic inductances and resistances of the capacitors and to other inherent parameters.
Coming up in Part 3: Voltage splitters based on power audio amplifiers.
About the author
Petre Petrov has worked as a researcher and assistant professor in Technical University (Sofia, Bulgaria) and as a lecturer and expert lecturer in Kingdom of Morocco. Now he is working as electronics engineer in private sector in Bulgaria.