An amplifier includes: a single-stage or multiple-stage variable gain amplifier that amplifies an input signal with a controlled gain; a AGC control circuit that detects the peak level of a signal outputted from the variable gain amplifier in the final stage, converts the resultant signal to a digital signal, and outputs an AGC control signal for controlling the gain of the variable gain amplifier based on the converted digital signal; an EVR control circuit that outputs an EVR control signal according to a signal of setting an attenuation value or an amplification value for EVR inputted from an electronic variable resistor control terminal; and a gain control circuit that controls the gain of the variable gain amplifier in accordance with at least one of the AGC control signal and the EVR control signal. The occurrence of “popping” sounds caused by differences in DC voltage due to switching between an AGC circuit and an electronic variable resistor circuit can be suppressed.
1. An amplifier comprising:
a single-stage or multiple-stage variable gain amplifier that amplifies an input signal with a controlled gain;
an AGC control circuit that detects the peak level of a signal outputted from the variable gain amplifier in the final stage, converts the resultant signal to a digital signal, and outputs an AGC (Automatic Gain Controller) control signal for controlling the gain of the variable gain amplifier based on the converted digital signal;
an EVR control circuit that outputs an EVR (Electronic Variable Resistor) control signal according to a signal of setting an attenuation value or an amplification value for EVR inputted from an electronic variable resistor control terminal; and
a gain control circuit that controls the gain of the variable gain amplifier in accordance with at least one of the AGC control signal and the EVR control signal.
2. The amplifier according to claim 1 , wherein the gain control circuit switches between the AGC control signal and the EVR control signal in accordance with a switching control signal supplied from outside of the circuit, thereby controlling the gain of the variable gain amplifier in accordance with any one of the signals.
3. The amplifier according to claim 1 , wherein the gain control circuit comprises:
a signal converter that converts the AGC control signal to a gain control signal for controlling the gain of the variable gain amplifier; and
an adder that adds the gain control signal outputted from the signal converter and the EVR control signal, and
the gain of the variable gain amplifier is controlled based on a signal obtained by adding the gain control signal and the EVR control signal.
4. The amplifier according to claim 1 , wherein the AGC control circuit comprises:
a detection circuit that detects the peak level of a signal outputted from the variable gain amplifier in the final stage and converts the resultant signal to a digital signal; and
a counter circuit that outputs the AGC control signal based on the digital signal outputted from the detection circuit.
5. The amplifier according to claim 1 , wherein the variable gain amplifier comprises:
an operational amplifier in which a non-inverting input terminal is supplied with the input signal via a resistor; and
a variable resistor that is connected at one terminal to the non-inverting input terminal and is connected at the other terminal to a reference voltage terminal, and
an inverting input terminal of the operational amplifier is connected to an output terminal thereof, so that the gain is controlled by the ratio of resistance values of the resistor and the variable resistor.
6. The amplifier according to claim 5 , wherein the variable resistor comprises:
a series circuit of plural resistors; and
switching transistors, each of which is connected at one terminal to one of nodes between the respective resistors of the series circuit and is connected at the other terminal to one end of the series circuit, and
the gain control circuit controls switching of ON and OFF of each of the switching transistors.
7. The amplifier according to claim 6 , wherein the back gates of the switching transistors are grounded via a diode.
8. The amplifier according to claim 1 , wherein the variable gain amplifier comprises: an operational amplifier in which an inverting input terminal is supplied with the input signal via a resistor and a non-inverting input terminal is connected to a reference voltage terminal; and a variable resistor that is connected between the inverting input terminal and an output terminal, so that the gain is controlled by the ratio of resistance values of the resistor and the variable resistor.
9. The amplifier according to claim 8 , wherein the variable resistor comprises: a series circuit of plural resistors; and switching transistors, each of which is connected at one terminal to one of nodes between the respective resistors of the series circuit and is connected at the other terminal to one end of the series circuit, and
the gain control circuit controls switching of ON and OFF of each of the switching transistors.
10. The amplifier according to claim 9 , wherein the back gates of the switching transistors are grounded via a diode.
11. The amplifier according to claim 1 , wherein plural variable gain amplifiers are connected in a cascade arrangement, and the gain of each of the variable gain amplifiers can be controlled independently.