Power detector

Power indicator - Effect detector

The present invention relates to a power detector in which the signal to be detected is applied to one input of the detector element and has a circuit for compensating for the variation of the threshold voltage of the detector element caused by temperature changes.

Power detectors are used, among other things, in the output control circuits of RF signal amplifiers. Previously, power amplifiers had only one output power, which was adjusted to the desired level during manufacture. Today, it is common for radio channels in radio communication systems to be used more than once. Therefore, in order not to mix two radio signals on the same frequency, it is desirable to be able to select multiple output power levels. Such a system is implemented, for example, by using a detector diode to detect a signal proportional to the output power and the output power to a comparator controlling the current amplifier, which current amplifier controls the gain of the RF signal amplifier. The output power of the stage amplifiers is proportional to the control current input to the amplifiers.

A rectifier diode is usually used to detect the power level of a high-frequency power amplifier, and a high-frequency voltage, for example an RF voltage, to be detected is applied to the anode, and a cathode is supplied with a rectified voltage proportional to the input voltage. The voltage from the detector is applied to an amplifier and comparator, which controls a current amplifier, from which current is directed to the downstream amplifiers. Thus, in addition to the power detector, a circuit for generating and regulating the current controlling the amplifiers is required for power control.

In order for the above-mentioned diode detector to operate even at low power levels, the rectifier diode must be biased in some way because the diode has a certain threshold voltage. This threshold voltage is also temperature dependent, so in biasing

39110 the change in threshold voltage caused by temperature must also be taken into account.

To compensate for the change in the threshold voltage caused by the change in temperature, it is known to use a second diode which is biased in the forward direction and from which a voltage is applied via a resistor to the detector diode as a bias voltage. The variation of the threshold voltage of the detector diode as the temperature changes is compensated for when the voltage across the other diode changes in the same way if the diodes are at the same temperature and have the same temperature coefficient.

The object of the present invention is to implement a temperature-compensated detector which at the same time acts as an amplifier. The invention is characterized in that the detector element is a transistor and that the circuit includes a current source which compensates for changes in the threshold voltage of the detector transistor according to temperature and by means of which the detector transistor is biased.

The transistor acting as a detector is biased by means of a power supply to act as a class B amplifier and thus as a half-wave rectifier. The detector is temperature compensated and acts as an amplifier that can amplify either current or voltage, depending on where in the circuit the output pin is placed.

The invention will be described in more detail with reference to the accompanying figures, in which Figure 1 shows the connection of an active power detector according to the invention and Figure 2 the input / output voltage curve of the power detector.

In the connection of the active power detector n according to the invention shown in Fig. 1, the current source forming the bias circuit is implemented around two transistors V2, V3. By means of the feedback formed by transistors V2 and V3

69110 the voltage across the resistor R3 is kept such that the transistor V3 and thus also the transistor VI remains at the limit of the active range. The quiescent current is thus almost zero and the transistor is biased to act as a class B amplifier. Thus, the detector transistor VI is biased to operate even at low RF powers. To achieve optimal balance, transistors VI and V3 must be of the same type. The temperature compensation of the transistor VI is implemented by means of the transistor V3. When the transistors VI and V3 are of the same type and are at the same temperature, then as the temperature changes, the threshold voltage of the transistor V3 changes in the same proportion as the threshold voltage of the transistor VI. Thus, the current supplied by the power supply also changes slightly as the temperature changes and keeps the detector transistor VI at the same operating point, i.e. at the limit of the active range at different temperatures. The amount of current supplied by the power supply, i.e. to the base of the transistor VI, can be determined by means of the resistor R1. Thus, if the resistors R2 and R5 are selected to be equal, the magnitude of the base current of the transistor VI can be substantially affected by the resistor R1. If the resistor R1 is selected to be substantially larger than the resistors R2 and R5, the output signal is almost zero when the RF signal is zero, i.e., the transistor VI remains at the limit of the active range until an RF signal is applied to the base.

The measured RF signal is applied to the base of the transistor VI as shown in Fig. 1, and the detected signal, which is a direct voltage, is taken out of the emitter of the transistor VI. When a sinusoidal signal is applied to the base of transistor VI, transistor VI acts as a half-wave rectifier, i.e. conducts with positive half-cycles and outputs zero current with negative half-cycles. In this case, since the transistor VI conducts the RF signal with positive half cycles, it charges the capacitor C2, whereby a direct voltage DC-OUT proportional to the high frequency signal is output. Access of the RF signal to the bias circuit is prevented by means of impedance Z1 and capacitor C1. The impedance Z1 can be a strip or a resistor. The function of the incoming capacitor C3 of the RF signal is to connect the high frequency signal to the detector transistor VIe.

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The function of the capacitor C4 connected to the supply voltage is to filter out interference from the supply voltage.

Figure 2 shows the relationship between the input voltage RF-IN and the output voltage DC-OUT of an active power detector according to the invention. The state of the detector transistor VI can be clearly seen from the curve. When the input signal RF-IN applied to the base of the transistor VI is -20 dBm, i.e. 0.1 mV, the output voltage DC-OUT given by the transistor is almost zero and increases as the input voltage RFIN increases. Thus, transistor VI only begins to conduct when a signal is applied to the base because it is biased to operate in class B.

The active power detector according to the invention can also be used as a voltage amplifier. This is done by taking the output from the collector of the transistor VI and not from the emitter. In this case, the voltage gain is proportional to the ratio of the resistors R4 / R5. When the detector is used as a current amplifier, the resistor R4 is in principle not needed, since its only function in this case is to limit the collector current of the transistor VI. Resistor R4 is placed in the circuit for safety so that the circuit can be used as a voltage amplifier if necessary by changing the position of the output terminal to the collector of transistor VI.

The active power detector according to the invention is buffered and withstands load better than the passive power detector. As a result, the circuit around the detector required to control the power amplifiers of the stage is simplified, thus saving cost and space in the manufacture of the power control circuit. In addition, the active power detector can be used to amplify either current or voltage, depending on the application, so that the range of use of the detector according to the invention is wider than that of the passive detector.