In the broadband communication system such as a TV tuner, a highly linear variable-gain low noise amplifier is arranged in upstream of a mixer. Generally, the highly linear variable-gain low noise amplifier is implemented according to a current steering topology.
FIG. 1A is a schematic circuit diagram illustrating a conventional highly linear variable-gain amplifier. This highly linear variable-gain amplifier is disclosed in IEEE J. Solid-State Circuits, vol. 26, pp. 1673-1680, November 1991. As shown in FIG. 1A, a first transistor Q1 and a second transistor Q2 are connected with each other to define a differential pair. The bases of the first transistor Q1 and the second transistor Q2 serve as the differential signal input terminals of the amplifier to receive an input signal vi. The first terminals of two emitter resistors Re are respectively connected to the emitters of the first transistor Q1 and the second transistor Q2. The second terminals of the two emitter resistors Re are collectively connected to a node “a”. A current source (Is) is interconnected between the node “a” and a ground terminal Gnd.
The bases of a third transistor Q3 and a fourth transistor Q4 serve as the gain control terminals of the amplifier for receiving a current steering control signal Vctrl. The collector of the third transistor Q3 is connected to a voltage source Vcc. The emitter of the third transistor Q3 is connected to the collector of the first transistor Q1. A first collector resistor Rc1 is interconnected between the voltage source Vcc and the collector of the fourth transistor Q4. The emitter of the fourth transistor Q4 is connected to the collector of the first transistor Q1. The base of a fifth transistor Q5 is connected to the base of the fourth transistor Q4. The base of a sixth transistor Q6 is connected to the base of the third transistor Q3. The collector of the sixth transistor Q6 is connected to the voltage source Vcc. The emitter of the sixth transistor Q6 is connected to the collector of the second transistor Q2. A second collector resistor Rc2 is interconnected between the collector of the fifth transistor Q5 and the voltage source Vcc. The emitter of the fifth transistor Q5 is connected to the collector of the second transistor Q2. The collectors of the fourth transistor Q4 and the fifth transistor Q5 serve as differential signal output terminals of the amplifier for generating an output signal vo.
The current source (Is) may provide DC bias voltages to all transistors of the amplifier. The two emitter resistors Re may offer good linearity of the amplifier. In addition, the resistance of the first collector resistor Rc1 is identical to that of the second collector resistor Rc2.
In response to a change of the current steering control signal Vctrl, the bias currents flowing through the third transistor Q3, the fourth transistor Q4, the fifth transistor Q5 and the sixth transistor Q6 are varied, and thus the gain value of the amplifier are adjustable. Moreover, the above amplifier may acquire a high gain control range.
Generally, the noise figure (NF) of the highly linear variable-gain amplifier is varied with the gain value. FIGS. 1B and 1C are schematic diagrams illustrating the relationship between the gain and the noise figure (NF) of the conventional highly linear variable-gain amplifier. As can be seen from FIGS. 1B and 1C, as the gain value of the amplifier is increased, the noise figure is decreased. Whereas, as the gain value of the amplifier is decreased, the noise figure is increased. That, when the gain value of the amplifier is adjusted according to the current steering control signal Vctrl, the noise figure is increased at nearly the same rate as the gain value is decreased.
In a case that the magnitude of the input signal vi is very low, the gain value of the amplifier is usually adjusted to the maximum value, and thus the noise figure is not too large. In a case that the magnitude of the input signal vi is relatively larger, the gain value needs to be reduced. In this situation, the noise figure of the amplifier is increased, and the signal is also amplified. In other words, the magnitude of the output signal vo allows for providing a sufficient signal-to-noise ratio (SNR). However, in the broadband communication application, the interference and noise are sometimes greater than the useful signal. For preventing the electronic components of the amplifier from entering the saturation region, the gain value of the amplifier needs to be decreased. If the increase of the noise figure is too obvious, however, the magnitude of the output signal vo fails to provide a sufficient signal-to-noise ratio (SNR), and thus the baseband circuit fails to effectively restore the signal. That is, when the amplifier has a low gain, low noise figure (NF) is very critical.
FIG. 2 is a schematic circuit diagram illustrating another conventional highly linear variable-gain low noise amplifier. The highly linear variable-gain low noise amplifier is disclosed in for example U.S. Pat. No. 6,100,761. As shown in FIG. 2, a first transistor 1Q1 and a second transistor 1Q2 are connected with each other to define a differential pair. The base of the first transistor 1Q1 is connected with a base voltage Vb through a first base resistor 1Rb1. The base of the second transistor 1Q2 is connected with the base voltage Vb through a second base resistor 1Rb2. The bases of the first transistor Q1 and the second transistor Q2 serve as the differential signal input terminals (IN+ and IN−) of the amplifier.
The first terminals of two variable emitter resistors (1Re) 40 are respectively connected to the emitters of the first transistor 1Q1 and the second transistor 1Q2. The second terminals of two variable collector resistors (1Rc) 30 are respectively connected to a collector voltage Vc. Moreover, the collectors of the first transistor Q1 and the second transistor Q2 serve as the differential signal output terminals (− OUT +) of the amplifier.
In the amplifier of FIG. 2, the gain value of the amplifier is adjusted by changing the resistances of the variable emitter resistors (1Re) 40 and the variable collector resistors (1Rc) 30. The changes of the variable emitter resistors (1Re) 40, however, may deteriorate the linearity of the amplifier.