Stabilisation of quiescent current has been appearing as a real difficulty for years.

Proper offset supply of quiescent current gives a compromise between the energy efficiency of power amplifier and its THD factor.

The very low THD factor known to be characteristics of A-class amplifiers can also be attainable in AB-class under the condition that regulation of quiescent current is maintained not only in the steady state but also dynamically over the whole range of working current and voltage amplitudes. In the latter case the circuit governing the offset polarisation should be fast enough i.e. inertialess.

There are known different polarisation circuits mostly based on thermal feedback with temperature sensing elements placed on the heat sink together with output stage transistors. However, the internal temperature levels of transistor junctions continually vary in time along with output signals and these fluctuations are not detectable using normal heat sensors. Therefore even A-class amplifiers have difficulty in maintaining transistors at their optimum operating points.

There are also known dynamic types of polarising circuits which enhance offset current during the switching transition between the output transistors as these frequently used by Pioneer Company. Mainly from Russian literature one knows the DC offset current regulators which sample the current dynamically during the zero crossings.

The circuits with thermal feedback are normally very slow. On the other hand the circuits with dynamic sensing in the near zero region are additional sources of distortions and should not be used in the low THD designs. Practical realisations of quiescent current supply appear very complex, element-sensitive, and, what is worse, ineffective.

An ideal solution to these problems would be a circuit which can sense offset current instantly, however, it should be made independently on the output signal contents. Instantly, means that it can not be current sensing performed during certain maximum or zero transitions and this should not exhibit high inertia resulting in high time canstant like do all the thermal feedbacks. A good base for a close-to-ideal offset supply of quiescent current could be a DC feedback regulator provided that it is equipped with a detector of polarisation current whose operation is independent on instantaneous currents in the output stage and its output is not altered by any signals which appeared prior to present moment.

Typical push-pull output stages operate the way that positive and negative half - waves of output currents and voltages are formed in separate transistors in the output stage and both these half-waves are summed in the load. Therefore when a sinusoid is being amplified each of the output transistors conduct sequentially only half-waves of that sinusoid plus a quiescent current which, to a first approximation, should be left unchanged. As a matter of fact these half-waves, positive or negative, creates also a DC current component, as in rectifiers, which can certainly be much greater than the set supply of quiescent offset DC current.

Only a proper sensing of the latter, i. e. close to the physical ideal, which does not depend on the actual or previous signal contents can solve the problem of low THD factor in transistor power amplifiers.

The means of extraction of the offset quiescent current from final stage of transistor power amplifier is that one forms the difference and the sum signals from two voltages : The first voltage comes from a first current sensing resistor introduced in series into high current path of the first output stage transistor which is conducting current within the first of two half-periods of sinusoidal signal which is the output signal of the amplifier or its harmonic component.

The second voltage comes from a second current sensing resistor introduced in series into high current path of the second output stage transistor which is conducting current within the second of two half-periods of the output signal.

From these two voltages one obtains sum and difference signals. Next, from the sum signal one obtains absolute value (module) signal which is to be subtracted from the difference signal if the value of voltage at the first current sensing resistor is a positive one, or added if that voltage is a negative one.

As a result of these operations one finally obtains a signal which is proportional to the DC offset quiescent current not dependant on the signal content in the amplifier load.

The circuit for extracting the offset quiescent current from output stages of transistor power amplifiers is that each of the current sensing resistors connected in series with respective output transistors is connected with the input of a first analog adder which develops a summary signal and, at the same time, with a second analog adder which develops a difference signal of voltages taken from both current sensing resistors.

The output of the first adder is connected to the absolute value function circuit whose output is connected to the third adder. The second input of that third adder is connected with output of the second adder which develops the difference signal.

The circuit according to the present invention features practically zero- inertia detection of quiescent current extracting it from other signals which occure in power amplifier. It is to be stressed that this could not have been achieved up to the date and even in 1998 (see : International Product Exporter May/June 98) such solutions to this problem have been proposed for audio amplifiers like for instanee speeial "Thermally Reaetive Advanced Instantaneous Transistor"with built-in elements importing thermal stability.

The subject of the invention is explained by drawings where Fig. l shows schematic diagram of a push-pull power amplifier accomplished with the use of complementary pair of power transistors and Fig. 2 shows time shapes of voltages appearing across the first and across the second current sensing resistor, waveforms of the sum and difference signals and time-sape of the module signal.

Block diagram which illustrates the means according to the present invention is shown in Fig. 3 and the Fig. 4 shows an exemplary realisation of the circuit for quiescent current sensing. A proposal for feedback regulation of quiescent current is shown in Fig. 5.

In order to realise the means according to the present invention (see Fig. 1) one should introduce conventional current sensing resistor RA 9RB into the high current path of both power transistors TA and TB. It is assumed to be obvious that current sensing resistors can also be placed the way as RAA and RBB are placed <BR> <BR> in Fig. 1. It is also assumed that both power transistors can be of any type or can be substituted by a plurality of power transistors connected in order to increase e. g. power capacity of such substitutes.

In Fig. 2 and Fig. 3 from voltage UA taken across current sensing resistor RA connected in series into high current path of power transistor TA which conducts current in first of two half-periods of sinusoidal output signal (or sinusoidal component of it) and from voltage UB taken across second sensing resistor RB connected in series into a high current path of second power transistor TB which conducts current during second half-period of said output signal one creates the sum signal (UA+UB) in the adder 1 and the difference signal (UA_UB) in the adder2. From the (UA +UB) signal one processes an absolute value (module) signal IUA+UBI = ABS (UA+UB). Absolute value signal is formed in the function circuit 4 and along with the difference signal it is sent to the third adder 3. The absolute value signal IUA+UBI is then subtracted from the difference signal (UA- UB) if the latter is of positive value or it is added to if the latter is a negative one. <BR> <BR> <P>In the present example (Fig. 1-4) it has been assumed half periods across RA to be positive and therefore the adder 3 should perform subtraction. And consequently, if the signal appearing at RA was negative the adder 3 should perform addition.

Output signal of adder 3 readily gives signal Upp proportional to the DC value of quiescent current which flows through power transistors TA and TB.

The circuit for extracting the quiescent offset current from final stage of transistor power amplifier according to the present invention has two resistors RA and RB each of those connected in series into the high current path of respective power transistors TA,TB. Each of current sensing resistors RA and RB is connected with the first adder 1 performing the (UA+UB) summation of the voltages from both resistors and also connected with the second adder 2 performing subtraction UA_UB of those voltages. Output of the second adder 2 is connected with the first input of the third adder 3. Output of the first adder 1 which provides the (UA+UB) signal is sent to the function circuit 4 which performs an analog function of absolute value (module) providing the signal IUA+UBI. This module signal JUA+UBI Is then sent to the second input of third adder 3 which performs the following operation : Upp = (UA UB) +/- IUA+UBI An example of practical realisation of the circuit according to the present invention is shown in Fig. 4 where a block 5 has been realised with the use of operational amplifiers 6 and 7 and it plays the role of first adder as well as it performs determination of the absolute value JUA+UBI. Amplifier 8 develops the Upp voltage equal to the sum or difference between the (UA_UB) and the absolute value IUA+UBI The circuit for regulation of the quiescent current is shown in Fig. 5 in which the quiescent current extraction circuit described in the present invention has been used as a quiescent current sensor in the feedback controlled quiescent current regulator. Transistor 9 is the regulating element which is controlled by means of opto-coupler 10 and resistor 11 and the error amplifier is realised using amplifier 12 whose second input is connected to voltage reference UREF. The value of UREF can be chosen by the manufacturer as it sets an optimal value of quiescent current. Only now tins current will not be dependent on the other voltages and currents in power amplifier and it will not depend on environmental conditions too. The circuit is a simple one and can be integrated into a single chip along with other features like e. g. servo-DC and SOA controllers.