Switched gain low noise amplifier

[0002] This invention relates in general to communications electronics and, more specifically, to low noise amplifiers used in communications downconverters.

[0003] The sensitivity of a typical communications receiver (i.e., the ability of the receiver to recover information in the presence of noise) depends, in part, on the noise figure of the receiver. This (the noise figure) is the excess noise generated by the receiver and translated to its input, as compared to the thermal noise (expressed in decibels). The noise figure of the receiver depends strongly on the performance of a low noise amplifier found at the front of the receiver.

[0004] An example of such an amplifier 10 is shown in FIG. 1. As shown, the low noise amplifier 10 includes a common-emitter gain stage that comprises an input capacitor 12 for receiving an RF signal, an inductor L_ethat provides emitter degeneration and helps set the input impedance R_INof the amplifier 10, and a bipolar transistor 14. In addition, the amplifier 10 may include a cascode stage for minimizing Miller capacitance and improving reverse isolation of the amplifier 10. The cascode stage comprises an AC grounding capacitor 16 and another bipolar transistor 18 that together receive a bias voltage V_bias, and a load element Z_Cthat typically includes an inductor and resistor in parallel. The amplifier 10 may also include a bypass MOS transistor 20 that can bypass the common-emitter gain stage when the RF signal is strong.

[0005] It is desirable for the input impedance of the amplifier 10 to be 50Ω, both in its “normal” or “high-gain” mode (i.e., when the bypass transistor 20 is off) and in its “bypass” or “low-gain” mode (i.e., when the bypass transistor 20 is on). In the normal mode, the input impedance R_INof the amplifier 10 is determined in accordance with:

[0006] R_IN≈r_π+ρβL_e  (1)

[0007] where r_π is the small-signal base-emitter input resistance looking into the base of the bipolar transistor 14, ρ is the dominant pole associated with β, and β is the common-emitter current gain. The value of the inductor L_eis selected from this equation.

[0008] There are several drawbacks to the design of this amplifier 10. For example, the required gain for the single-stage amplifier 10 is significant. Also, the inductor L_eand the inductor in the load element Z_Care typically off-chip, which allows these inductors to couple to input pins and destabilize the amplifier 10. Further, the amplifier 10 can be difficult to tune because the input impedance R_IN, the input compression point, and the noise figure of the amplifier 10 are set by transistor geometry, inductor degeneration, and quiescent current, all of which are heavily interdependent. In addition, it is difficult to get the bypass mode input impedance, which is set by the MOS transistor 20, to match the normal mode input impedance R_IN, which is set as discussed above.

[0009] Accordingly, there is a need in the art for an improved low noise amplifier.

[0010] An inventive amplifier (e.g., a low noise amplifier) includes a first gain stage that receives an input signal (e.g., an RF signal) and, in response, generates an amplified signal in, for example, the high-gain mode of the amplifier and a low-gain signal in the bypass mode of the amplifier. In the high-gain mode, switching circuitry outputs the amplified signal, and a second gain stage generates an output signal in response. Also, in the high-gain mode shunt circuitry feeds the output signal back to the first gain stage, and in the bypass mode feeds the low-gain signal forward as the output signal.

[0011] This inventive amplifier has improved matching of its bypass mode input impedance and its high-gain mode input impedance, improved stability due to the feedback through the shunt circuitry in the normal mode, and improved reverse isolation due to the two stage amplifier design.

[0012] In a further embodiment, the amplifier described above is incorporated into a communications downconverter that also includes an image rejection filter and a mixer.

[0013] In another embodiment, an inventive low noise amplifier generates an output voltage from an RF signal. The amplifier includes a first gain stage having a bipolar transistor that receives the RF signal at its base and generates an amplified signal at its collector in the normal mode of the amplifier and a low-gain signal at its emitter in the bypass mode of the amplifier. Also, switching circuitry includes first and second bipolar transistors with their emitters coupled to the collector of the first gain stage bipolar transistor, and an inverter coupled to the bases of the first and second bipolar transistors that receives a bypass signal and, in response, switches on the second bipolar transistor and switches off the first bipolar transistor for common-emitter operation of the first gain stage in the normal mode, and switches off the second bipolar transistor and switches on the first bipolar transistor for emitter-follower operation of the first gain stage in the bypass mode. In addition, a second gain stage includes a bipolar transistor with its base coupled to the collector of the second bipolar transistor of the switching circuitry that receives the amplified signal and generates the output voltage at its collector in the normal mode. Further, shunt circuitry includes an impedance network coupled between the collector of the second gain stage bipolar transistor and the emitter of the first gain stage bipolar transistor that feeds the output voltage back to the first gain stage in the normal mode and couples directly the emitter voltage at the first gain stage to the output of the second gain stage in the bypass mode.

[0014] In still another embodiment, an input signal is amplified by first receiving the input signal with a first gain stage. Then, in the normal mode, the first gain stage is switched into common-emitter operation, an amplified signal is generated from the input signal using the first gain stage, an output signal is generated from the amplified signal using a second gain stage, and the output signal is fed back to the first gain stage through shunt circuitry. Also, in the bypass mode, the first gain stage is switched into emitter-follower operation, a low-gain signal is generated from the input signal using the first gain stage, and the output signal is generated from the low-gain signal using the shunt circuitry.

[0015] FIG. 1 is a circuit schematic illustrating a conventional low noise amplifier;

[0016] FIG. 2 is a circuit schematic illustrating a low noise amplifier in accordance with this invention; and

[0017] FIG. 3 is a block diagram illustrating a downconverter incorporating the low noise amplifier of FIG. 2.

[0018] As shown in FIG. 2, an improved low noise amplifier 30 in accordance with this invention includes a first gain stage 32 that comprises an input capacitor 34 for receiving an input RF signal, a bipolar transistor 36, and an impedance element Z_e1. A second gain stage 38 includes another bipolar transistor 40, a load element Z_c2, and an impedance element Z_e2. Also, a shunt circuit 42 includes an impedance network Z₀and an AC coupling capacitor 44. Further, a switching block 46 includes a differential bipolar pair 48 and 50, an inverter 52, a load element Z_c1, and another AC coupling capacitor 54.

[0019] It will be understood by those having skill in the technical field of this invention that the amplifier 30 may be implemented using transistor technologies other than those illustrated in FIG. 2.

[0020] In the “normal” or “high-gain” mode of operation, a bypass signal turns the bipolar transistor 50 on and the bipolar transistor 48 off, causing the first gain stage 32 to operate as a cascode common-emitter gain stage. In this normal mode of operation, the shunt circuit 42 feeds a portion of the output voltage V_OUTback to the first gain stage 32.

[0021] Also, in the “bypass” or “low-gain” mode of operation, the bypass signal turns the bipolar transistor 50 off and the bipolar transistor 48 on, causing the first gain stage 32 to operate as an emitter follower stage. In this bypass mode of operation, the shunt circuit 42 feeds the output of the first gain stage 32 forward to the second gain stage 38 as the output voltage V_OUT.

[0022] The improved low noise amplifier 30 has improved tuneability for matching the bypass mode input impedance to the normal mode input impedance, improved stability due to the shunt feedback, and improved reverse isolation due to the two stage amplifier design. In simulations, 20 dB of gain, low noise operation, and 50Ω matched input impedance with simple matching have been achieved (for both modes of operation).

[0023] As shown in FIG. 3, a downconverter 60 includes the low noise amplifier 30 of FIG. 2, an image rejection filter 62, and a mixer 64.

[0024] Although this invention has been described with reference to particular embodiments, the invention is not limited to these described embodiments. Rather, the invention is limited only by the appended claims, which include within their scope all equivalent devices and methods that operate according to the principles of the invention as described.