Monolithic AC Level Detector

This invention relates to detector arrangements generally and more particularly to an improved AC level detector especially suited for adaptation in integrated circuit chip form.

AC level detectors form a part of many kinds of electronic circuits and have always posed a problem due to the conflicting demands of high accuracy and economy of parts. With the advent of monolithic circuit design, the use of resistors in circuits has become undesirable due to the chip area required for them and the power lost in them. For low voltage battery powered devices, such as pocket pagers, these considerations become even more important. Previous designs for AC level detectors have required a number of resistors in an attempt to reduce sensitivity to supply voltage variation, which resulted in added inaccuracies due to resistor tolerances.

It is the object of the present invention to provide an improved AC level detector adapted for implementation in integrated circuit chip form.

It is a more particular object to provide an improved detector of the foregoing type which is characterized by a high degree of accuracy but which requires a minimum of power and a minimum of chip area.

The above objectives are achieved in an intergrated circuit chip in accordance with the present invention by the use of current source biasing the current mirrors to eliminate dependance on resistor-determined bias levels. The core of the level detector is comprised of three transistors, each having a separate current source. Being on a single chip, all have identical physical characteristics with the exception of junction area. The first transistor is diode connected and the voltage reference level which is established across it is coupled to the emitter-base junction of the second transistor. The second transistor thus tracks the voltage of the first transistor exactly but since it has either a much smaller junction area or a larger current source the second transistor cannot support the available current and the excess current from the second source is diverted to the base of the third transistor for controlling its conductivity. The current in the second transistor thus provides a current reference level. Under normal no-signal conditions, the third transistor is in saturation and the output voltage across the third transistor is essentially zero. When an AC signal is added to the voltage reference level on the base of the second transistor it causes the current in the latter to vary around the current reference level. When the current in the second transistor reaches a predetermined level, the third transistor is driven to cut off and the supply voltage appears across the output.

The threshold level of the input voltage, which also determines the length of the output pulses, can be varied by the manufacturing design parameters of the chip, or can be varied by adjusting the current sources with digital control inputs.

FIG. 1 is a circuit diagram of a conventional current mirror.

FIG. 2 is a circuit diagram of the basic elements of a level detector according to the present invention.

FIG. 3 is a block diagram of a typical application including the level detector circuit.

FIG. 4 is a chart of waveforms at strategic points in the circuit of FIG. 2 .

Referring now more particularly to the drawing, wherein like parts have like numerals throughout, FIG. 1 shows a conventional current mirror 10 as known in the art. The term "mirror" is used to signify that the current in one portion of the circuit faithfully tracks the current in another portion. A diode-connected transistor 11 ("diode" hereinafter) is connected in series with a constant current source 12. The diode voltage at point B is applied to the base of transistor 13. If the current in the diode 11 is I, the current in the transistor 13 will be NI where N is the ratio of the junction area of the transistor 13 to the junction are of the diode 11.

FIG. 2 shows an arrangement constructed in accordance with the present invention wherein a current mirror such as shown at 10 may be connected to control another transistor such as at 14, each diode and transistor having its own current source, indicated generally at 12, 15 and 16, respectively. As in the circuit of FIG. 1, the voltage reference of the diode 11 is applied to the transistor 13. In the preferred embodiment, current sources 12 and 15 are identical and the junction area of transistor 13 is less than the juction area of transistor diode 11. Thus, the transistor 13 will not support the full current of its source 15. It is to be understood as also within the scope of the invention to have the two referenced junction areas identical, but have the current source 15 capable of supplying more current than the source 12. In either case, the excess current from the source 15 will be diverted to the base of transistor 14 which may thereby be driven into saturation.

FIG. 3 includes the circuit arrangement of FIG. 2, now designated as reference numeral 17 with a typical environment as shown in block diagram form. As indicated, input circuit 18 has been added to the circuit of FIG. 2 and consists of an isolating resistor 18a and capacitor 18b. A current reference source 19 is coupled to the current sources 12, 15, 16. The sensitivity of the level detector can be controlled by electronically or manually switching an additional current source into or out of the collector circuit of either transistor 11 or transistor 13.

In this exemplary application, a signal received by an antenna 20 of an electrical device, such as for example, a pocket pager, is demodulated in an FM receiver portion 22. The resulting audio signal, which might consist of a sequence of five or six audio frequency tones is connected to an active filter 23. Since the pager is looking for one specific sequence of tones, the filter will initially be looking for the first tone of that sequence. If the tone is present in the filter output 24 with sufficient amplitude, the AC level detector 17 according to the present invention will provide output to the decoder 25 through an integrating network 26 and a latching circuit 27. The purpose of the integrating network 26 is to prevent one strong noise pulse from activating the decoder. The latching circuit 27, once activated by a desired tone, provides a sustained output in case the received signal is lost too soon to activate the decoder 25. A first decoder output 28 is coupled to reset the active filter 23 and a second decoder output 30 will instruct the active filter to look for the second tone of the desired sequence. This procedure may be reiterated, if necessary, for second, third and fourth tone-initiated signals, utilizing decoderfilter connections 31, 32 and 33. Each consecutive tone must be received within a predetermined time interval for proper circuit activation. The fifth and sixth tone-initiated signals are coupled by means of connections 34 and 35 to a pager alert 36 which notifies the user that his personal code has been received. Typically, the function of a toneonly pager as here referenced is to alert the user that a message is awaiting him at a predetermined telephone address.

In FIG. 4, the waveforms for voltage and current at various critical points in circuit 17 are shown in relation to a signal input voltage 38 being applied to the input terminal 24. A time line 39 indicates the beginning of an AC input. A voltage waveform 40 represents the voltage reference level at point B and remains essentially constant, as does a current waveform 41, isolated from the input signal 38 by the resistor 18a. The waveform 41 represents the current in the diode 11. A voltage 42 at point C is essentially the sum of waveforms 38 and 40 and appears on the base of the transistor 13. A current 43 is the baseemitter current of transistor 13. A voltage 44, point D, appears on the collector of transistor 13 and the base of transistor 14.

A current waveform 45, shown as a solid line, is the collector current of the transistor 13, and dotted current waveform 46 is the base-emitter current of the transistor 14. A voltage waveform 47 is the output voltage at the point E. A current waveform 48 is the collector current of the transistor 14.

Operationally, when there is no input signal applied to the terminal 24, all other voltages shown remain unchanging at the respective predetermined levels as indicated by the first portion of each waveform, to the left of the time line 39. When the AC input signal 38 is applied to the level detector circuit via the terminal 24, the signal is superimposed on the reference voltage 40 as shown in the second portion of 42 and the current 43 through the base-emitter of transistor 13 is controlled accordingly. As this current rises, the voltage 44 begins to fall and when it reaches zero or essentially zero, as at time 49, transistor 13 reaches saturation as shown by its collector current 45. Transistor 14 is cut off and the voltage 47 across the transistor 14, and thus on the output terminal 21, jumps to a value approximating the supply voltage. The pulses of waveforms 47 are coupled to the integrator circuit 26 as described hereinbefore and, if of proper amplitude and duration, activates the decoder 25 and the decoding procedure continues.

The theory of operation can best be understood by consideration of the current-voltage characteristics of a semiconductor junction. In the conduction region, the current flowing through the junction is related to the voltage across it by the diode equation: ##EQU1## Where: ##EQU2## and is a function of several physical parameters related to the structure of the junction. On a single IC chip, these parameters are the same for all devices of the same type. Thus, J.sub.s is a constant for devices with the same geometry: A is the area of the semiconductor junction, V is the voltage across the junction, q is the electronic charge, k is Boltzmann's constant and T is the temperature is absolute degrees.

Now, from equation (1), it follows that if the two matched devices have different junction areas, the currents through the two devices are given by: ##EQU3## where I.sub.1, I.sub.2, V.sub.1, V.sub.2 are the junction currents and voltages. Further, if these two devices are connected together so that they have the same junction voltage, we have: ##EQU4## Thus, the currents through the junctions ae directly related to the junction areas. This relationship between current and area is the basis of the current mirror frequently used in bipolar integrated circuits and shown in FIG. 1. In this circuit, in which the emitter base junctions of transistors 11 and 13 are connected in parallel and where transistors 13 has N times the emitter-base junction area of transistor 11, the current I that is forced through the diode-connected transistor 11 is "mirrored" as a current flow NI in 13. That is, 13 could support a collector current of NI if it were available. Since the area-current relationship (4) is valid for any value of N, it is also valid for N = 1/2. In this case transistor 11 has twice the area of transistor 13. For N = 1/2, the transistor 13 in the current mirror shown in FIG. 1 can support or "sink" a current of 1/2I. If a current source of greater than 1/2I were connected to the collector of transistor 13, the transistor would not be able to sink the current in excess of 1/2I. Thus, as in FIG. 2 where current sources 12 and 15 (each = I) are connected to both transistors 11 and 13, transistor 13 can only sink 1/2I and the remainder of current from current source 15 flows into the base of transistor 14 and forces it to be saturated, i.e., the collector of transistor 14 is forced to be near ground.

Now, if an AC signal is capacitively coupled to the base of the transistor 13 as shown in FIG. 3, the current in the transistor 13 will vary directly with the input signal level. If the peak current in the transistor 13 reaches I, then the transistor 14 will have no base current and its output will be off. The input signal required to cause the transistor 14 to switch off is determined by the circuit parameters and the integrated circuit geometries. From equations 3, the incremental voltage required to increase the current in the transistor 13 by a factor of M is: ##EQU5## or:

for:

thus, whenever the AC input signal exceeds 0.026 1n (1/ N volts, the output of the detector circuit changes state, and this output signal can be processed by additional circuitry to implement a tone detector, etc.

Thus there is provided, according to the invention herein disclosed, an adjustable and highly accurate AC level detector in integrated circuit form. The accuracy is, in large parts, due to the matched characteristics of transistors formed on one integrated circuit chip. By the use of transistors instead of resistors on the chip as current sources, not only is the accuracy of the detector improved, but the chip area required is greatly reduced along with the power losses in the current sources.

Although the invention has been shown in a specific embodiment for purposes of illustration, no limitation is to be inferred therefrom. It is intended to include all variations and modifications falling within the spirit and scope of the invention.