Gain control circuit and method

Abstract: A circuit and method for controlling current gain in variable gain and automatic gain control amplifiers or attenuators in high frequency communication systems. A transistor through which the current is provided always operates at the same DC bias, independent of gain control variations. The input signal is provided to a base of the transistor whose collector provides an output current whose gain is to be controlled. An AGC current for adjusting the output current gain and a DC bias current are provided to the base of the transistor through a multi-transistor circuit.

Description: The present invention relates to circuits for electronically controlling signal level, and in particular to a gain control circuit for variable gain and automatic gain control amplifiers or attenuators in high frequency communication systems. Many circuits in communication system circuits require devices for controlling signal level. Variable gain and automatic gain control amplifiers and attenuators are used for this purpose. A known device for controlling signal level in communication systems is the differential pair attenuator. An output from a collector of either transistor in the attenuator is a scaled replica of the current input at the pair's emitters, where the amount of attenuation is a function of the applied differential voltage. In such devices an input signal and a DC bias are both scaled. However, at high attenuation the reduced DC bias affects operation of the device, causing the bandwidth to decrease and the current gain to drop. The known arrangement avoids this problem by cascading several attenuator stages, each with less attenuation than a single attenuator. Cascading introduces other problems, such as the need to stack the cascaded stages which impairs low supply voltage operation, or the use of current mirrors or level shifters to connect stages which add to system supply current requirements and may reduce system bandwidth. The present invention avoids many of these problems by operating a transistor through which the input signal is provided with a DC bias which is independent of gain control variations. The input signal is provided to a base of the transistor whose collector provides an output current whose gain is to be controlled. An AGC current for adjusting the output current gain and a DC bias current are provided to the base of the transistor through a multi-transistor circuit which ensures that the DC bias is substantially independent of variation of the output current gain. An object of the present invention is to provide a gain control circuit and method in which a transistor through which the input signal is provided operates with a DC bias which is substantially independent of gain control variations. Subject of the invention is a gain control circuit having a gain control circuit having a cell connected between a first potential and ground, said cell comprising a first transistor having a first terminal connected to an output for the cell, a grounded second terminal and a base connected to an input for the cell; a first pair of serially connected transistors connected between the first potential and ground, a first node between said first pair being connected to said base of said first transistor; a first and second AGC current source for adjusting the gain of an input current for the cell; a bias current source for providing a DC bias current; a second pair of serially connected transistors which are connected between said bias current source and ground, each of said second pair of transistors having its base connected to a first terminal thereof, said first AGC current source being connected to a second node between said second pair of transistors, and a base of one of said second pair of transistors being connected to a base of said first one of said first pair of transistors; a sixth transistor connected between said second AGC current source and ground, and having its base connected to a base of said second one of said first pair of transistors and to a first terminal of said sixth transistor, so that a DC bias provided to said base of said first transistor is substantially independent of variations of gain control. Another subject is a method of controlling current gain a method of controlling current gain comprising the steps of providing an input current to a base of a first transistor having a first terminal for providing an output current whose gain is to be controlled and a grounded second terminal providing: a first pair of serially connected transistors with a first node between said first pair of transistors connecting to the base of said first transistor; a second pair of serially connected transistors which are diode-connected and having a second node between said second pair of transistors; a DC bias current to the base of the first transistor through said second transistor and said first node so that the DC bias is independent of variation of the output current gain; a first and second AGC current for adjusting the output current gain connecting said first AGC current to said second node between said second pair of serially connected transistors and connecting said second AGC current to a sixth diode-connected transistor which has its base connected to the base of said second one of said first pair of transistors. The invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is circuit diagram of an embodiment of the basic cell of the present invention. Figure 2 is circuit diagram of cascaded cells of an embodiment of the present invention. Figure 3 is circuit diagram of a further embodiment of cascaded cells of the present invention. Figure 4 is circuit diagram of an embodiment of the present invention for a differential device. Figure 5 is circuit diagram of a second embodiment of the present invention for a differential device Figure 1 shows an embodiment of the basic cell 10 for a gain control circuit that includes a first transistor Q1 having a collector connected to an output I OUT for the cell, a grounded emitter, and a base connected to an input I iN for the cell. Only a small percentage of the current applied to the base of the transistor Q1 is absorbed in the base, with most being absorbed in the emitter of the transistor Q2 at node N1. Transistor Q2 thus converts the input current to a voltage, and the voltage to an output current. A first pair of serially connected transistors Q2 and Q5 may be connected between a potential V DD and ground and have a node N1 between transistors Q2 and Q5 which is connected to the base of transistor Q1. An AGC current source I AGC may provide a current for adjusting the gain from I IN to I OUT (while two current sources I AGC are shown, one such source may suffice), and a bias current source I BIAS may provide a DC bias current. A second pair of serially connected transistors Q3 and Q4 may be connected between I BIAS and ground, and each may have its base connected to its collector. I AGC may be connected to a node N2 between the second pair of transistors Q3 and Q4, and a base of transistor Q4 may be connected to a base of transistor Q2. A sixth transistor Q6 may be connected between I AGC and ground, and have its base connected to its collector and to a base of transistor Q5. In operation, a DC bias provided to the base of transistor Q1 is independent of variation of current gain. For example, the transistors may be bipolar junction transistors which may be either PNP or NPN, or they may be MOS transistors. The bias currents and AGC currents may be the same or different, may vary from cell to cell in a cascade of plural cells. An understanding may be facilitated by the following mathematical description of the operation of the circuit and method. If all of the transistors are the same size and matched, V BEQ1 + V BEQ2 = V BEQ3 + V BEQ4 I C1 I C2 = I C3 I C4 I OUT (I AGC + I IN ) = I BIAS (I AGC + I BIAS ) As will be appreciated, I IN = I BIAS + I SIG , where I SIG is the input signal level. Then, I OUT = ( I BIAS + I AGC ) I BIAS I AGC + I BIAS + I SIG I OUT = I BIAS 1 + I SIG I BIAS + I AGC = I BIAS 1 + x = I BIAS (1 - x + x 2 - x 3 ) where x = I SIG I BIAS + I AGC I OUT = I BIAS y + (1 - y ) + x where y = I AGC I BIAS + I AGC and I IN = [(1 - y) + x](I BIAS + I AGC ) Thus, I OUT ≃ I BIAS (1-( ai )+( ai ) 2 -( ai ) 3 ) where a ≡ 1 I BIAS + I AGC ; i ≡ I SIG Output current I OUT has the same DC bias, and an AC portion which is phase shifted 180' and is scaled by the factor, I BIAS I BIAS + I AGC Since I AGC may be positive or negative, the cell is capable of gain or attenuation. The basic cell 10 will operate at low supply voltages, such as two transistor V BE 's (approximately 1.6V), and the total supply current is comparable to cascaded differential pairs. Further, since the transistor Q1 through which the signal passes always operates at the same DC bias which is independent of the attenuation factor, the bandwidth is also substantially independent of the current gain and cell 10 provides a wide, constant bandwidth over the entire attenuation range. The bias current may also be used to vary gain. For example, if the value of I BIAS in Figure 1 decreases, attenuation will be increased without changing I AGC . Figure 2 shows plurality of the cells 10 that can be cascaded with no level shifting and no DC blocking capacitors or other matching networks between cells as is typical of the prior art. The output of one cell 10 may be provided as an input to the following cell 10'. Cascaded cell 10' may vary signal level with completely separate components, such as shown in Figure 2. If the output of cell 10 becomes the input to a second identical cell 10, then I OUT has the same DC bias but the AC portion is scaled by a factor, If the output of the second cell 10 is the input to a third identical cell 10, then I OUT has the same DC bias but the AC portion has a 180° phase shift and is scaled by a factor, With two identical cells 10, an attenuation range from 0 to -45dB is attainable. Operation of a gain control circuit is desirably linear, and to this end it has been found that the best linearity is obtained with an even number of cascaded cells. For linear operation, the ratio x (from Equation 5) of signal current I SIG to the sum of I BIAS and I AGC is desirably small, with the worst case being when there is no AGC (maximum gain.) However, alternate cells have a tendency to cancel distortion, and with two cascaded cells the second cell cancels the distortion from the first cell when the AGC is zero, y = 0 (from Equation 6). Therefore, the worst case overall is at some intermediate level of gain. For three cells, distortion is about the same as for a single cell with only a slight improvement when I AGC equals I BIAS , y= 0.5. Thus, linearity is best with an even number of stages. Figure 3 illustrates cell 10" that shares the biasing portion (I BIAS , Q4, Q3 and Q6) with another cell 10, provided the currents I BIAS and I AGC are the same and the transistors are the same size. The basic cell 10 may also be adapted for use in a differential cell 20, such as illustrated in Figure 4. Three transistors Q7-9 are added to replicate the three transistors Q1, Q2 and Q5 of cell 10'" in order to provide the second input and output for cell 20. The circuit of Figure 5 illustrates a further embodiment in which the emitters of the transistors Q1 and Q7 are connected in common and to ground through a resistor R1. A second resistor R2 having twice the resistance value of R1 is connected between the emitter of Q4 and the collector/base of Q3. The tying together of the emitters forms a virtual ground and essentially none of the current from the input signal passes through R1. This tends to stabilize the operating point of the transistors Q1 and Q7 with regard to the mismatch between the transistors Q1 and Q7, and it tends to balance the signal levels. The disadvantage is that distortion has its onset at a lower value and thus distortion may slightly increase as the a.c. portion of the signal increases. The invention finds application in a variety of devices, including AGC devices ( i . e ., devices with electronically variable gain), gain control amplifiers and attenuators, and other similar devices. The invention is particularly adaptable for use in high frequency communication systems. A circuit and method for controlling current gain in variable gain and automatic gain control amplifiers or attenuators in high frequency communication systems. A transistor through which the current is provided always operates at the same DC bias, independent of gain control variations. The input signal is provided to a base of the transistor whose collector provides an output current whose gain is to be controlled. An AGC current for adjusting the output current gain and a DC bias current are provided to the base of the transistor through a multi-transistor circuit.

Claims: A gain control circuit having a cell connected between a first potential and ground, said cell comprising a first transistor (Q 1 ) having a first terminal connected to an output for the cell, a grounded second terminal and a base connected to an input for the cell; a first pair of serially connected transistors (Q 2 , Q 5 ) connected between the first potential and ground, a first node (N 1 ) between said first pair (Q 2 , Q 5 ) being connected to said base of said first transistor (Q 1 ); a first and second AGC current source for adjusting the gain of an input current (I IN ) for the cell; a bias current source for providing a DC bias current (I BIAS ); a second pair of serially connected transistors (Q 3 , Q 4 ) which are connected between said bias current source and ground, each of said second pair of transistors (Q 3 , Q 4 ) having its base connected to a first terminal thereof, said first AGC current source being connected to a second node (N 2 ) between said second pair of transistors (Q 3 , Q 4 ), and a base of one (Q4) of said second pair of transistors being connected to a base of said first one (Q 2 ) of said first pair of transistors; a sixth transistor (Q 6 ) connected between said second AGC current source and ground, and having its base connected to a base of said second one (Q 5 ) of said first pair of transistors and to a first terminal of said sixth transistor (Q 6 ), so that a DC bias provided to said base of said first transistor is substantially independent of variations of gain control. A circuit as claimed in claim 1 characterized by plural ones of said cell in a cascade, in which an output of a first one of the cells becomes an input for a second one of the cells, preferably an even number of said plural cells. A circuit as claimed in claim 1 characterized in that the cell further comprises a seventh transistor (Q 7 ) having a first terminal connected to a second output (I OUT +) for the cell, a grounded second terminal and a base connected to a second input (I IN -) for the cell; a third pair of serially connected transistors (Q 8 , Q 9 ) connected between the first potential and ground, a third node (N 3 ) between said third pair (Q 8 , Q 9 ) being connected to said base of said seventh transistor (Q 7 ); and a base of said first one (Q 8 ) of said third pair being connected to said base of said first one (Q 2 ) of said first pair (Q 2 , Q 5 ), and a base of a second one (Q 9 ) of said third pair (Q 8 , Q 9 ) being connected to said base of said second one (Q 5 ) of said first pair (Q 2 , Q 5 ). A circuit as claimed in any of one of claims 1 to 3 characterized in that all of said transistors are NPN bipolar transistors of substantially the same size, or in which all of said transistors are PNP bipolar transistors of substantially the same size, or in which all of said transistors are MOS transistors of substantially the same size. A circuit as claimed in any one of claims 1 to 4 suitable for use in a gain control amplifier, or a gain control attenuator, or in an automatic gain control system. A method of controlling current gain comprising the steps of providing an input current (I IN ) to a base of a first transistor (Q 1 ) having a first terminal for providing an output current (I OUT ) whose gain is to be controlled and a grounded second terminal providing: a first pair of serially connected transistors (Q 2 , Q 5 ) with a first node (N 1 ) between said first pair of transistors connecting to the base of said first transistor (Q 1 ); a second pair of serially connected transistors (Q 3 , Q 4 ) which are diode-connected and having a second node (N 2 ) between said second pair of transistors; a DC bias current (I BIAS ) to the base of the first transistor (Q 1 ) through said second transistor (Q 2 ) and said first node (N 1 ) so that the DC bias is independent of variation of the output current gain; a first and second AGC current (I AGC ) for adjusting the output current gain connecting said first AGC current to said second node (N 2 ) between said second pair of serially connected transistors (Q 3 , Q 4 ) and connecting said second AGC current (I AGC ) to a sixth diode-connected transistor (Q 6 ) which has its base connected to the base of said second one of said first pair of transistors (Q 2 , Q 5 ).

Inventors: PRENTICE JOHN S, PRENTICE, JOHN S.
Applicants: HARRIS CORP, HARRIS CORPORATION
Application year: 1997
Main classification: H03G1
IPC codes: H03G1, H03F3, H03G3