Crossover Circuit for Use in Automatic Gain Control Systems

The subject matter of this disclosure is related to the subject matter of U.S. patent application, Ser. No. 220,607, entitled "LARGE SCALE, SINGLE CHIP INTEGRATED CIRCUIT TELEVISION RECEIVER SUBSYSTEM" filed concurrently herewith which is assigned to Motorola Inc.

1. Field of the Invention

The present invention relates to automatic gain control systems and more particularly to a changeover circuit for providing output signals indicative of the level of an automatic gain control (A.G.C.) input signal supplied thereto.

2. Background of the Prior Art

If the amplitude of the composite video signal developed in a television receiver is allowed to vary significantly, a strong incoming signal may cause the video amplifiers of the receiver to become overloaded resulting in cross modulation and clipping of the synchronizing components of the signal, while a weak incoming signal may cause the output of the video amplifiers to be too low to provide proper picture reproduction. In addition, unwanted variations of contrast may result from a video signal which is changing in amplitude. Therefore, in all television receivers some form of automatic gain control (A.G.C.) is used to keep the detected video constant despite variations in the level of the composite radio frequency (RF) input signal. The most common system is to sample and make the synch, the tip of the composite video signal equal to a direct current (DC) reference voltage. This is normally effected by means of a gated amplifier which derives a voltage (stored on a capacitor) the level of which first gain reduces the IF amplifier as the input signal increases, the RF gain remaining unchanged. After some predetermined point, gain reduction of the IF stage ceases and the IF gain remains constant with further gain reduction being made by reducing the gain of the RF stage of the receiver. For example, U.S. Pat. No. 3,697,883, which is incorporated herein by reference, describes such a system.

The overall characteristic of the aforementioned system enables the television receiver to have the best possible signal to noise ratio at low signal levels because the RF gain is fixed and optimized for good noise performance while at high signal levels, where noise performance is not as important, overloading of the receiver and hence intermodulation problems are prevented by the gain reduction of the RF stage. However, the changeover from affecting the gain reduction of the IF stage to the gain reduction of the RF stage should not result in both the RF and RF amplifiers being acted on simultaneously for a large part of the AGC curve, or conversely there should be no gap between the two regions, i.e., IF gain reduction region and the RF gain reduction region. Both of these faults lead to large changes in AGC loop gain and difficulties in controlling the loop response to disturbances.

Some contemporary changeover systems suffer in that the point of changeover between IF and RF gain control is not well-defined and hence suffer from the aforedescribed faults. Thus, there is a need for a circuit of the type being discussed which makes the possibility of either of the aforementioned effects almost zero by reducing the changeover region to negligible proportions. Such a circuit should also fulfill the needs for being suitable to be manufactured as an integrated circuit in a large scale integrated circuit comprising a television reveiver.

Accordingly, it is an object of the present invention to provide an improved automatic gain control changeover circuit.

Another object of the invention is to provide an automatic gain control changeover circuit for providing output voltages which are indicative of an applied input signal being below and above a predetermined level respectively.

Still another object of the present invention is to provide an automatic gain control changeover circuit for utilization in a television receiver for varying the gain of the IF and RF stages thereof in response to a gain control signal being applied thereto.

In accordance with the above and other objects, there is provided a changeover circuit comprising a first circuit loop and a second circuit loop. The first circuit loop is in a responsive state whenever the magnitude of an applied input signal is below a threshold level set by the magnitude of a reference bias signal supplied thereto to produce an output therefrom which is indicative of the level of the input signal. The second circuit loop coupled with the first circuit loop is held in a non-responsive state until such time as the input signal magnitude becomes greater than the threshold level to produce at that time an output which is indicative of the level of the input signal. As the magnitude of the input signal becomes greater than the threshold level, the first circuit loop becomes non-responsive to maintain the output therefrom substantially constant.

It is a feature of the present invention that the output signals from the first and second circuit loops are provided in response to an automatic gain control signal for varying the gain of the IF and RF stages respectively of a television receiver.

FIG. 1 is a schematic diagram of a changeover circuit of the preferred embodiment of the invention;

FIG. 2 shows waveforms useful in describing the operation of the circuit shown in FIG. 1.

Turning to FIG. 1, there is shown a unity gain closed loop feedback changeover circuit 10 of the present invention which is adapted to receive an automatic gain control signal at the input 12 thereof. The AGC input signal supplied at input 12 may be developed across an AGC capacitor as aforedescribed as generally is known. In response to the AGC input signal, changeover circuit 10 provides at respective outputs 14 and 16 a pair of output signals which may be utilized for example for controlling the gain of the IF amplifier stage and RF stage respectively of a television receiver. For instance, not intending to limit the scope of the present invention, changeover circuit 10 could be utilized in conjunction with the gate AGC circuit of the aforementioned U.S. Pat. No. 3,697,883 to provide the gain control signals to the video IF amplifier and the RF amplifier and converter circuits respectively. Additionally changeover circuit 10 may be utilized in the AGC and RF AGC circuits disclosed in the aforereferenced U.S. patent application, Ser. No. 220,607, entitled "LARGE SCALE, SINGLE CHIP INTEGRATED CIRCUIT TELEVISION RECEIVER SUBSYSTEM" which is incorporated herein by reference. Changeover circuit 10 includes a comparator amplifier 18 comprising NPN transistors 20 and 22 which are coupled in a differential amplifier configuration with the common emitters thereof coupled to current source 24. The collectors of transistors 20 and 22 are returned directly and via resistor 26 respectively to power supply terminal 28 at which is supplied a source of operating potential V+; current source 24 being returned to power supply terminal 30 which receives a ground reference or -V operating potential. A bias reference signal VR is applied at terminal 37 to the base of transistor 34 which is differentially connected with companion PNP transistor 36. The common connected emitters of transistor 34 and 36 are returned to power supply conductor 28 via resistor 38 with the respective collectors thereof coupled to power supply terminal 30. The base of transistor 36 is coupled to the collector of transistor 22 at node 48. The emitters of transistor 34 and 36 are coupled to the base of transistor 49 which is connected in cascade, at output 14, to resistor 42 and via resistor 46 to the second input, the base of transistor 22, of amplifier 18. As shown in phantom, transistor 49 may be a dual emitter transistor with the output of the first circuit loop being taken across resistor 57, at output 14', instead of across resistor 42. In this manner output 14' would be buffered from any changes in signal level across resistor 42 when the first circuit loop is in a non-responsive state.

For all magnitudes of the AGC input signal applied to terminal 12, the first input of amplifier 18, being less than a threshold level set by VR, the first circuit loop comprising transistors 22, 36 and 49 is in a responsive state whereby the output signal at terminal 14 follows changes in the AGC input signal and is indicative of the level thereof. Hence, as shown in FIG. 2, as the AGC input signal increases, the output at terminal 14, waveform portion 58, increases in a linear fashion. The output at terminal 14, for instance, can be utilized to provide the gain reduction function for the IF amplifier stages of the television receiver as aforementioned.

A second circuit loop comprising transistors 22, and transistor 44 is normally in a non-responsive state whenever the first circuit loop is in a responsive state, i.e., with the AGC input signal less than the threshold level transistor 44 is in a non-conductive state because the base-to-emitter junction is zero biased by the first circuit loop comprising transistors 49 and 36 which are coupled between the base of transistor 44 and, via resistor 46, to the emitter of transistor 44. At the crossover point, when the AGC input signal approaches the threshold level set by VR, transistor 36 will begin to turn off since its emitter is biased at a level equal to VR+V.sub.be, where V.sub.be is a base-to-emitter voltage drop. Any further increases in the AGC input signal causes the level at node 48 to increase rapidly to turn off transistor 36 rendering the first circuit loop non-responsive to further changes in the AGC input signal. As the AGC input signal increases beyond the threshold level the second circuit loop comprising transistors 22, and 44 becomes responsive to provide at output 16, via current turnaround circuit 50 (including diode 52 and PNP transistor 54) which is indicative of the further increase in the input signal as is shown by waveform 68. Hence, for example the output signal appearing at output 16, across resistor 56, which increases and is indicative of the applied input signal when the input signal is greater than the reference bias signal VR, can be utilized as the RF gain stage control function. With the second circuit loop being responsive, that is, transistor 44 being rendered conductive, the voltage across resistor 42 and therefore the voltage level at output terminal 14 is held at substantially the value VR, a constant. Thus, in the region above VR the voltage output at terminal 14 is held constant, waveform portion 62, while the output signal at output 16 rises as the input signal moves more positive than VR.

Hence, what has been described above, is a changeover circuit which is suitable to be utilized in a large scale integrated circuit for a television receiver to provide the IF and RF gain functions for controlling the gain of those respective stages in response to a gain controlled input signal applied to the input thereof. In the region where the AGC input signal is below the reference bias signal VR the IF voltage rises with increases in the input signal until the input signal reaches the level of the bias reference signal. Thereafter, the IF voltage remains constant whereas the RF AGC drive voltage rises as the input signal moves more positive than the reference bias signal. The aforedescribed circuit prevents both the IF and RF amplifier stages of the television receiver from being acted on simultaneously while maintaining no gap between the two regions, i.e., the IF gain control region and the RF gain control region. The break point or changeover point at which the IF AGC gain signal is held constant and the RF AGC voltage is allowed to increase with the AGC input signal to the circuit is made adjustable by changing the value of the bias reference signal which is externally generated to the circuit such that the IF voltage output is maintained constant and does not change with variations in temperature. The changeover region is forced to be very small due to the large gain around the amplifier loop of the changeover circuit.