The present invention relates to amplifiers and more particular to amplifiers employing shunt feedback.
The traditional series shunt feedback architecture illustrated in FIG. 1 has been widely hailed as the preferred architecture for Low Noise Amplifiers (LNAs) requiring a well defined input impedance. However combining series shunt feedback with Gilbert""s current steering techniques for providing an Automatic Gain Control (AGC) function presents certain difficulties.
The following equations describe how the input impedance Rin of the traditional series shunt feedback amplifier is set by a negative feedback amplifier:
Rf=Rs*(1+A)
Rin=Rf/(1+A)
where Rf is the effective feedback impedance, Rs is the source impedance, and |A| is the magnitude of the gain of the amplifier. It will be appreciated that as long as Rf remains constant, then so does the input impedance Rin. However, if the amplifier has a varying gain, then the input impedance Rin will also change.
A potential solution to this problem is offered by the architecture of FIG. 2, where the input impedance of an amplifier arrangement is set by an input stage comprising an amplifier which has a fixed gain. The output of the input stage feeds a variable gain amplifier stage. This solution is not optimal however, since the input stage has a fixed gain and the input signal to the variable gain amplifier stage is therefore higher than the input signal to the amplifier arrangement as a whole. This creates a problem in signal handling requirements and may result in a power in-efficient solution.
In view of the disadvantages noted above, existing amplifier arrangements may not meet the strict requirements of modem electronic systems, such as interactive cable television equipment, in which a range of high performance RF products are required ranging from mixer oscillator circuits to LNAs with AGC and loop through circuits.
It is an object of the present invention to provide an amplifier arrangement having a stable input impedance. It is a further object of the present invention to provide a low noise amplifier which is power efficient. These and other objects are achieved at least in part by an amplifier arrangement in which the amplifier stages thereof are operated in current mode.
According to a first aspect of the present invention there is provided an AC voltage amplifier arrangement comprising:
an input stage having an input, an amplifier connected in series shunt feedback mode, and an output; and
a second stage having an input coupled to the output of said input stage, and an output,
wherein said input and second stages are arranged to operate in current mode.
Preferably, said second stage is a variable gain amplification stage.
Preferably, the voltage amplifier arrangement is arranged to amplify radio frequency voltage signals.
Preferably, said amplifier of the input stage is a transconductance amplifier. More preferably, the transconductance amplifier comprises a bipolar junction transistor configured as a common emitter stage, or a differential pair of bipolar junction transistors configured as common emitter stages.
Preferably, the arrangement comprises a unity gain buffer amplifier coupled in the series shunt feedback loop of the input stage.
Preferably, said second stage comprises a current steering arrangement arranged in use to steer a proportion of the current, provided to the input of the second stage, to an output load, whereby an output voltage is developed across said load and is available at said output of the second stage. The current steering arrangement may be a Gilbert steering stage. More preferably, said current steering arrangement comprises a pair of bipolar junction transistors having their emitters coupled together and to said output of the input stage. The current steering arrangement may comprise two pairs of bipolar junction transistors in a differential arrangement, coupled respectively to differential outputs of the input stage.
The second stage may be arranged to be controlled by an Automatic Gain Control (AGC) signal in order to maintain the output of the stage at a desired level. More preferably, where the stage comprises a pair (or pairs) of bipolar junction transistors, said AGC signal is applied to the bases of the transistors.
The second stage may comprise a mixer or a buffer.
According to a second aspect of the present invention there is provided a method of amplifying an AC voltage signal, the method comprising:
converting the voltage signal into a current signal;
amplifying said current signal to a desired level; and
converting the amplified current signal into an amplified voltage signal.
Preferably, the amplification factor by which the current signal is amplified is less than one.
Preferably, amplification of the current signal comprises splitting the current signal into at least two parts, with one of said parts being converted into the amplified voltage signal.