This invention relates to an intermediate frequency amplification circuit for use in a mobile radio station (unit), a cordless telephone set, and/or a telemeter system, and, more particularly, to an intermediate frequency amplification circuit which is capable of measuring or detecting a field strength.
A conventional intermediate frequency amplification circuit of the type described comprises an amplification part which is supplied with a circuit input signal to produce a circuit output signal subjected to amplification therein. In addition, a signal strength measuring part is included in the intermediate frequency amplification circuit to measure or detect a field strength of the circuit input signal.
More particularly, the amplification part of such an amplification circuit comprises a plurality of differential stages connected in cascade to one another. Each of the differential stages comprises a first pair of transistors having emitters coupled in common to each other. On the other hand, the signal strength measuring part measures the field strength by detecting a d.c. level of the circuit input signals. For this purpose, the signal strength measuring part comprises a plurality of rectification circuits for rectifying output signals of the differential stages to produce rectified signals, respectively, and capacitors coupled to the respective rectification circuits so as to add the rectified signals to one another. Each of the rectification circuits is implemented by a plurality of transistors to carry out half-wave rectification.
With this structure, each capacitance of the capacitors in the rectification circuits inevitably becomes large with a reduction of an intermediate frequency necessary in the intermediate frequency amplification circuit. On manufacturing such an intermediate frequency amplification circuit by the use of a semiconductor integration technique, necessity of a large capacitance gives rise to an increase of a chip size. In order to reduce a chip size, it might be considered that the respective capacitors are connected as external capacitors outside of a chip. However, use of the external capacitors results in an increase of terminals derived from the chip and makes it difficult to apply an integration technique to such an amplification circuit. At any rate, the above-mentioned amplification circuit requires the capacitors equal in number to the rectification circuits.
Moreover, each rectification circuit has a bad temperature characteristic. In order to improve the temperature characteristic, complicated compensation must be carried out in each rectification circuit.
In Unexamined Japanese Patent Publication No. Sho 60-246108, an intermediate frequency amplification circuit is disclosed which comprises first through last full-wave rectification circuits connected in cascade to one another. Each of the full-wave rectification circuits comprise first and second differential amplifiers and third and fourth differential amplifiers connected to the first and the second differential amplifiers in series across a power source, respectively. Each of the rectified output signals is sent through a current mirror circuit to a current summation circuit to produce a field strength signal. The current summation circuit may include a single capacitor.
Incidentally, the first differential amplifier of each full-wave rectification circuit serves to supply an amplified signal to a next following one of the full-wave rectification circuits. As a result, a circuit input signal is supplied to the first full-wave rectification circuits and subjected to amplification through each of the full-wave rectification circuits to produce a circuit output signal through the last full-wave rectification circuit.
With this structure, it is possible to improve a temperature characteristic and to produce the field strength signal by the use of a single capacitor.
However, the intermediate frequency amplification circuit is disadvantageous in that a source voltage must comparatively be high because the first and the second differential amplifiers must be connected to the third and the fourth differential amplifiers in series across the power source. In fact, it is difficult to drive the amplification circuit at a voltage lower than 2 volts or so. This shows that power consumption becomes large in the intermediate frequency amplification circuit.