Broadband amplification (generally from less than 100 MHz to greater than 75 GHz) is desired for many applications, such as microwave and optical communication networks and test and measurement equipment. Gain of approximately 20–30 dB is desirable to obtain moderate power levels (typically about 10 to 30 dBm) for electrical signals. Distributed amplifier integrated circuits (“ICs”), particularly those using gallium arsenide (“GaAs”) and/or indium phosphide (“InP”) transistors, are often used as gain elements in these types of systems. Distributed amplifiers provide good input and output impedance matching and moderate output power over a wide frequency range.
Multi-stage amplifiers cascade a series of distributed amplifiers (“amplifier stages”), often provided as unpackaged semiconductor ICs (also known as “chips”), to achieve higher gain. Gain of 20 to 30 dB is often obtained with multi-stage amplifiers. The gain of the multi-stage amplifier depends on the gain per amplifier stage and number of amplifier stages in the cascade, as well as other factors. However, cascading amplifier stages can be difficult due to the incompatibility of the bias levels of the output of one amplifier stage with the input of the next amplifier stage.
An example of a distributed amplifier is an integrated, monolithic semiconductor traveling wave amplifier (“TWA”). Three to ten active gain cells distributed along two transmission lines on a semiconductor chip are typically used in a TWA. One transmission line is the input transmission line and the other transmission line is the output transmission line. Each of these transmission lines is terminated in a resistor having the same value as a characteristic impedance (e.g. 50 ohms) of the transmission line. These resistors are referred to as the input termination and output termination. The active gain cells are often FETs, which are arranged in a cascode configuration. GaAs depletion-mode FETs are often used as the gain cells in distributed amplifiers because they provide a good combination of gain, high-frequency performance, and power.
For purposes of this discussion, a cascode configuration of depletion-mode FETs can be represented as a single common source FET, since the electrical characteristics of the two are similar. If the distributed amplifier consists of 7 FETs, each having a width of 50 microns, the lumped equivalent will be represented as a single FET having a width of 350 microns.
The input signal to the distributed amplifier is coupled to the gate of the first FET (active gain stage), which is typically biased at a slight negative voltage (about −0.3 V) and draws a very small amount of current (typically<0.1 mA). The signal is amplified by the FET and the output signal is taken from the drain of the FET, which is typically biased at a positive voltage (about 4V) and draws a significant amount of current (typically 75 mA-500 mA or more). The output signal is coupled to the input of the next amplifier stage; however, the bias point (4V) of the drain of the first amplifier stage is not compatible with the bias point (−0.3V) of the second amplifier stage. Multi-stage amplifiers often use either DC blocks (capacitors) or level-shifting networks to resolve incompatible bias levels for the output of a preceding amplifier stage and the input of a following amplifier stage.
A common technique is to use a broadband blocking capacitor to isolate the output bias of one amplifier stage from the input bias of the next amplifier stage. Drain current is provided to the output of the first amplifier stage through a broadband inductor.
FIG. 1A shows a circuit diagram of a portion of a prior art multi-stage amplifier 10 using capacitively coupled amplifier stages 12, 14 (represented as FETs). A blocking capacitor 16 blocks DC from the drain 18 of the first amplifier stage 12 to the gate 20 of the second amplifier stage 14. Drain bias voltage VD is supplied to the drain 18 of the first amplifier stage 12 through an inductor 22. A large capacitor 30 (typically greater than 100 pF) coupled to the drain 12 through a 50 ohm output termination resistor 26. The combination of the capacitor 30 and output termination resistor 26 provides a good termination for the drain 12 at both low and high frequencies. The value of the output termination resistor is chosen according to a characteristic impedance of the system that the multi-stage amplifier 10 is intended for use in. A 50 ohm termination is appropriate for use in a 50-ohm system, but this characteristic impedance is chosen merely as an example for convenience of discussion. Resistors 32, 34 form a voltage divider for biasing the gate 22 of the second amplifier stage 14.
Unfortunately, the capacitor 16 blocks low-frequency and DC signals in addition to the drain bias voltage VD. Generally, a larger capacitor will couple lower frequencies; however, it is also more likely to have a lower self-resonant frequency. Similarly, the inductor 22 often has resonant frequencies that affect the signal between amplifier stages, and broadband resonance-free inductors are expensive.
FIG. 1B shows a circuit diagram of a portion of another prior art multi-stage amplifier 40 using capacitively coupled amplifier stages 12, 14. Rather than using a broad-band inductor (see FIG. 1A, ref. num. 22), bias current is provided to the first amplifier stage 12 through a 50 ohm output termination resistor 26 by a voltage supply 42. A blocking capacitor 16′ isolates the drain bias 42 of the first amplifier stage 12 from the gate bias 44 of the second amplifier stage 14. The drain current for the first amplifier stage 12 flows through the output termination resistor 26, which is usually integrated on the semiconductor chip of the first amplifier stage 12. The voltage drop across the output termination resistor 26 is typically about 5 Volts. This generates significant power (e.g. 5V*100 mA=0.5 W) and raises the operating temperature of the first amplifier stage 12. Higher operating temperatures often result in earlier failures, especially with physically small resistors. Unfortunately, physically large resistors do not work well at high frequencies due to parasitic reactance.
FIG. 1C shows a circuit diagram of a portion of yet another prior art multi-stage amplifier 46. Level-shift diodes D1, D2, D3, D4 are used to shift the bias level from the drain 18 of the first amplifier stage 12 to the gate 20 of the second amplifier stage 14. As in the multi-stage amplifier shown in FIG. 1B, the drain current flows from the drain bias 42 through the output termination resistor 26. The drain bias 42 also forward biases the level-shift diodes D1, D2, D3, D4. However, the level-shift diodes D1, D2, D3, D4 have parasitic inductances and capacitances that create resonances at high frequency signals. An optional current source 47 is included to insure that a few milliamps are pulled through the level-shift diodes D1, D2, D3, D4. This also insures that a selected amount of current is pulled through the input termination resistor 48.
A “speed-up” capacitor 49 provides a low-impedance path for high-frequency signals, typically above 500 MHz, between the drain 18 of the first amplifier stage 12 and the gate 20 of the second amplifier stage 14. Below about 50 MHz, the capacitor does not couple the drain 12 to the gate 22, and the signal from the drain 12 passes to the gate 22 through the level-shift diodes D1, D2, D3, D4. Coupling the first amplifier stage 12 to the second amplifier stage 14 as shown often results in higher gain at low frequencies, and lower gain at high frequencies, rather than flat gain across the bandwidth of the multi-stage amplifier 46.
Thus, it is desirable to couple amplification stages in a multi-stage amplifier to allow amplification of low-frequency signals without degrading the high-frequency performance of the distributed amplifier and at the same time to provide level shifting to resolve the incompatibility in biasing levels between the output of one amplifier stage and the input of the next amplifier stage. It is also desirable to provide drain current to an amplifier stage in a manner that avoids the disadvantages discussed above.