Low voltage IC for voice operated transceiver

A portable transceiver with a transmit/receive switch which includes an audio amplifier, level detector and comparator all direct coupled and included on a single integrated chip. The comparator includes one input that has a slow attack time and a fast decay time for differentiating between slow changing background noise and fast bursts of voice so that the transceiver is voice operated.

The present invention pertains to voice operated portable transceivers and, 
more particularly, to low votage operated integrated circuits for 
controlling transmit/receive switches in transceivers. 
BACKGROUND OF THE INVENTION 
Conventional portable transceivers, such as portable one-way radios, are 
controlled with a mechanical push-to-talk switch which must be depressed 
manually each time the operator wants to transmit. In many instances the 
manual operation is inconvenient. As a simple example, repairmen receiving 
instructions by radio may need both hands to do the repairs. Many other 
instances where handfree operation would be convenient, or even necessary, 
will be apparent to operators as well as those skilled in the art. 
Some speaker phones for telephones and the like contain circuits that are 
voice operated. However, these circuits are relatively complex and require 
large amounts of voltage and power. Clearly, such circuits could not be 
used in portable transceivers where the amount of voltage, power usage and 
size are crucial. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved portable 
transceiver with voice operated transmit/receive switch. 
It is a further object of the present invention to provide an improved low 
voltage operated integrated circuit for controlling a transceiver 
transmit/receive switch. 
It is a further object of the present invention to provide a low voltage 
operated integrated circuit for controlling and/or operating a transceiver 
transmit/receive switch having direct coupled circuits and no more than 
eight external connections. 
It is further object of the present invention to provide a low voltage 
operated integrated circuit for controlling and/or operating a transceiver 
transmit/receive switch, which integrated circuit includes external band 
width, gain, filter and trigger level adjustments. 
These and other objects are realized in a portable transceiver including a 
transmitter, a receiver and a transmit/receive switch constructed to 
receive audio input signals from a microphone or the like and to turn on 
the transmitter when the input signals being received include a voice 
signal. The transmit/receive switch includes a low voltage operated 
integrated circuit with a comparator therein for differentiating between 
relatively slow changing background noise and bursts of voice.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring specifically to FIG. 1, a simplified block diagram of portable 
transceiver 10 is illustrated. Transceiver 10 includes an antenna 12, a 
diplexer 14 or other device for allowing the use of a single antenna for 
transmitting and receiving, a receiver 16, a transmitter 18 and a 
microphone 20. In this preferred embodiment microphone 20 is connected to 
the signal input of transmitter 18 through a voice operated switch 22. 
Voice operated switch 22 includes an audio amplifier that supplies voice 
signals from microphone 20 directly to transmitter 18. Further, voice 
operated switch 22 senses the voice signal and actuates, or turns on, 
transmitter 18 in response thereto. In some types of transceivers the 
diplexer and other circuits may be operated by the push-to-talk switch and 
under such circumstances voice operated switch 22 will perform the 
switching function or may simply control the various switches that control 
these circuits. 
Referring specifically to FIG. 2, a preferred embodiment of voice operated 
switch 22 is illustrated in schematic form. The circuit of voice operated 
switch 22 is included in an integrated circuit on a semiconductor chip 
designated 25 by the enclosed in the broken line. 
Microphone 20 is connected through bypass capacitor 26 and resistor 27, 
which are located off chip 25, to a first external connection pad, 
labelled MIC IN, on chip 25. The MIC IN pad is connected directly to the 
base of PNP emitter-follower input stage 28 of an audio pre-amplifier 
generally designated 30. Pre-amplifier 30 includes differential pair of 
NPN transistors 32 and 33 with an active loading circuit including PNP 
transistor pair 34 and 35 and a current source, NPN transistor 37. This 
circuit is different than conventional circuits in that current source 
transistor 37 is operated with a very small V.sub.CE of below 
approximately 0.1 volts. This low operating voltage is due to the fact 
that the base voltage of transistor 32 is biased at approximately 0.7 
volts, which is the V.sub.be (base-emitter junction potential) of input 
stage 28. Input stage 28 receives current from current source 38. A small 
base biasing current of 1 uA is drawn through a resistor 39 to ground. The 
input DC voltage of pre-amplifier 30 is equal to the current flowing 
through resistor 39 times the resistance value. In this embodiment the 
value of resistor 39 is 20K ohms so that the input DC voltage is 2 
millivolts, or close to zero. The output DC voltage is set at 
approximately 0.62 volts by using a ten to one emitter area for transistor 
33. 
The base of transistor 33 is connected through an external connection pad, 
labelled NF, to an impedance 40, made up of a series connected resistor 41 
and capacitor 42 having one terminal connected to ground. External 
connection pad NF is connected through an external impedance (resistor) 45 
to an external connection pad labelled MIC OUT, which is in turn connected 
to the output of pre-amplifier 30. The MIC OUT pad is also connected 
through series connected capacitor 46 and resistor 47 to ground. The gain 
and band width of pre-amplifier 30 can be changed or varied by using 
differnt designs, including types and sizes of components, for impedances 
40 and 45 and capacitor 46. Resistor 47 is a load resistor for 
pre-amplifier 30. 
The output of pre-amplifier 30 is also connected directly to a level 
detector 50, which is in turn connected directly to a feedback amplifier 
55, including an attenuator circuit 56. Detector 50, amplifier 55 and 
attenuator circuit 56 form an automatic level control (ALC) circuit. Level 
detector 50 includes an NPN transistor 58, which provides the detector 
function and operates similar to a half-wave rectifier, and an emitter 
follower 59 connected to receive the output signals from pre-amplifier 30 
and supply them to the base of transistor 58. The emitter of transistor 58 
is connected to an external connection pad, labelled ALC, and through an 
external filter 60, including a resistor 61 and a capacitor 62 connected 
in parallel, to ground. Filter 60 determines the attack time constant of 
level detector 50, which is the resistance between the collector and 
emitter of transistor 58 times the capacitance of filter 60. Also, filter 
60 is external to the semiconductor chip 25 so that it can be easily 
changed if desired. 
Feedback amplifier 55 is designed similar to pre-amplifier 30 and includes 
a current source 65 with a V.sub.CE less than about 0.1 volts. Attenuator 
circuit 56, in this embodiment, is a single NPN transistor with the base 
connected to the output of feedback amplifier 55 and the collector 
connected to the base of input stage 28 of pre-amplifier 30. The single 
transistor is designed to obtain both low voltage operation (near 0 volts) 
and high attenuation ratios (&gt;50 dB). This is achieved by clamping the 
V.sub.CE of attenuator circuit 56 close to zero volts and by providing a 
16.times.emitter area NPN transistor to obtain high attenuation ratios. 
The attenuation ratio is equal to: 
EQU ALC attenuation ratio=20In[R.sub.ce /(R.sub.ce +R.sub.27)]dB. 
Thus, the attenuation range is changed by changing the value of resistor 27 
in the microphone circuit. 
The output signals from level detector 50 are connected directly through a 
level amplifier 70 to a noise monitor 72, and a comparator 75. Level 
amplifier 70 is a DC level amplifier similar to pre-amplifier 30, with the 
base biasing voltage of the differential amplifier being set at 
approximately a V.sub.be of 0.7 volts. Level amplifier 70 has a voltage 
gain of approximately 4 dB and supplies an output signal to an input of 
noise monitor 72 and, through a resistor 76, to a negative input of 
comparator 75. 
Noise monitor 72 includes a pair of PNP transistors, 78 and 79, 
differentially connected to provide an input for signals from level 
amplifier 70 at a base of transistor 78. The base of transistor 79 is 
connected to an external connection pad, labelled NSC (noise storage 
capacitor), which is in turn connected through a capacitor 80 to ground 
and through a resistor 81 to V.sub.CC. The base of transistor 79 is also 
connected to the emitter of PNP transistor 82, the collector of which is 
connected to ground and the base of which is connected to the emitter of 
transistor 79. The base of transistor 79 is further connected to a 
positive input of comparator 75. Noise monitor 72 has a slow attack time, 
determined by the values of resistor 81 and capacitor 80, and a fast decay 
time, determined by the R.sub.ce of transistor 82 and the value of 
capacitor 80. Since resistor 81 and capacitor 80 are located off of chip 
25, they are easily changed to adjust the attack and delay times. 
Comparator 75 is a two input differential amplifier with the output of 
level amplifier 70 connected to the positive input through noise monitor 
72 and to the negative input through resistor 76. Resistor 76 shifts down 
the level amplifier output by the voltage drop across resistor 76 so that, 
in the idle case, the positive input of comparator 75 has a higher DC 
voltage of I.sub.76 .times.R.sub.76. When the voltage of the negative 
input becomes higher than the voltage of the positive input, the 
conduction of comparator 75 reverses. This reversal turns on a PNP switch 
output transistor 85, the collector of which is connected to an external 
connection pad labelled SWITCH OUT/TRIGGER LEVEL ADJUST and through an 
external load resistor 86 to ground, to raise the output voltage and 
provide a switching action. 
The operation of level amplifier 70, noise monitor 72 and comparator 75 is 
based on the fact that the level change of background noise is always 
slower than voice, which appears as bursts. Therefore, if the attack time 
constant of noise monitor 72 is selected to approach the fundamental cycle 
of a background noise, the positive input of comparator 75 is always 
higher than the negative input until a voice signal is present. When a 
voice signal is present, the voltage rises at the negative input of 
comparator 75 faster than at the positive input because the signal at the 
positive input is delayed by charging of capacitor 80. When the voltage 
level at the negative input rises above the voltage level at the positive 
input, comparator 75 reverses conduction and drives switch output 
transistor 85 to produce the switching action. It should be noted that the 
switching action of transistor 85 can be used directly in place of a 
mechanical transmit/receive (push-to-talk) switch and/or it can be used to 
control other mechanical and/or electrical switching circuits utilized to 
replace the push-to-talk switch. 
A trigger level adjust circuit 88 is connected to external load resistor 86 
and to resistor 76. When the positive input of comparator 75 is high and 
transistor 85 is cutoff, resistor 86 is used to adjust the DC voltage at 
the base of PNP input transistor 89 of circuit 88. Transistor 89 
determines the amount of current flowing through NPN transistor 90, which 
is mirrored in a second NPN transistor 91 the collector of which is 
connected to resistor 76. The current flowing in transistor 91 determines 
the voltage drop across resistor 76 and, thus, the DC offset voltage 
between the inputs of comparator 75. Therefore, external resistor 86 can 
be used to adjust the trigger level of comparator 75 and the voice switch 
22. It will of course be understood that some or all of the external 
adjustable components, such as resistor 86, can be made continuously 
adjustable, if desired, by including a variable resistor or the like. When 
output transistor 85 is turned on, transistor 89 is in a cutoff state and 
resistor 86 is used only as a load resistor. 
A circuit 95 is connected to all of the circuits and is used as a current 
reference. The operation of circuit 95 is believed to be conventional and 
will not be discussed further herein. Also, all of the circuits described 
herein are designed to be direct coupled, i.e. no coupling capacitors, so 
that the entire circuit can readily be integrated on a single 
semiconductor chip, as shown. To further enhance the integration of the 
described voice switch, the output pads have been reduced to eight from 
the number twelve that would normally be needed. This reduction has been 
accomplished by the dual use of several pads as, for instance, the dual 
use of the SWITCH OUT/TRIGGER LEVEL ADJUST pad. Further, the circuit is 
designed to have a minimum number of external components. Also, the 
amplifiers and other circuits throughout the voice operated switch use 
differential stages with constant current sources operating under very low 
V.sub.CE (&lt;0.1 volts), which is important for low voltage, low power 
consumption and high frequency integrated circuit design. The V.sub.CE is 
dropped below 0.1 volts by using low current and low collector saturation 
resistances throughout the circuits. Low collector saturation resistances 
can be obtained, for example, by optimizing the NPN device layout. In this 
embodiment the semiconductor chip 25 is able to operate with a V.sub.CC 
under 1.8 volts. 
Thus, a new and improved voice operated switch has been disclosed which 
requires a low operating voltage and which is integrated onto a single 
semiconductor chip with a minimum number of external connection pads and 
external components. Because of the low operating voltage and small size, 
the switch can be used in portable transceivers and the like. Further, the 
switch is reliably controlled by voice signals so that it can be used in 
transceivers and the like in a "hands-free" operation. Also, band width, 
gain, filter characteristics and trigger levels can be easily adjusted 
external to the integrated circuit for different applications and areas of 
operation. 
While we have shown and described a specific embodiment of the present 
invention, further modifications and improvements will occur to those 
skilled in the art, we desire it to be understood, therefore, that this 
invention is not limited to the particular form shown and we intend in the 
appended claims to cover all modifications that do not depart from the 
spirit and scope of this invention.