Amplifiers – Signal feedback – Amplifier in signal feedback path
Reexamination Certificate
2000-09-28
2003-06-17
Mottola, Steven J. (Department: 2817)
Amplifiers
Signal feedback
Amplifier in signal feedback path
C330S086000
Reexamination Certificate
active
06580317
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a low-noise, broadband amplifier device having at least a first broadband amplifier element having negative feedback. The invention additionally relates to the use of the amplifier device.
Such an amplifier device is employed for example as an amplifier of an ultrasonic apparatus, magnetic resonance apparatus or of a radio-frequency measuring apparatus. In the amplifier device, an electrical signal which, for example, is generated by an ultrasonic transducer in response to a received acoustic signal, may have a very low signal level, and is in this case amplified for subsequent signal processing (not specifically described here). To ensure that the information content is not diminished unnecessarily this amplification should be effected with as little noise as possible.
The reference book U. Tietze, Ch. Schenk, “
Halbleiter
-
Schaltungstechnik” [Semiconductor circuitry
], Springer-Verlag, 9th edition, 1991, pages 44 to 49 and also 132 to 137 discloses various amplifier devices which include, in addition to at least one active amplifier element, such as a transistor or an operational amplifier, at least one further element for connecting up the amplifier element. In order to eliminate the nonlinearity of the active element and, in particular, also to set a defined gain and input impedance, use is often made here of the circuit principle of negative feedback.
The negative feedback is usually effected via a non-reactive resistor. However, since a non-reactive resistor generates thermal noise, the originally good noise properties of the active amplifier element are significantly impaired by the resistive negative feedback.
In order to avoid this negative effect of resistive negative feedback, inductive transformer-based negative feedback is provided instead in the case of the amplifier devices respectively described in the Company catalog of Adams-Russell Co. Inc. “RF&Microwave Signal Processing Components” in the section “Amplifier Application Note” on pages 20 and 21 and in U.S. Pat. No. 3,624,536. A very good noise behavior can thus also be achieved for the amplifier device overall, since a transformer is a very low-loss and low-noise element. However, a transformer is relatively expensive and also rather large, so that it can only be integrated with difficulty.
The technical paper “
Rauscharme Verstärkerschaltung” [
Low
-
noise amplifier circuit
], Neues aus der Technik, 1979, No. 3, 15.06.1979, p. 2 describes a low-noise amplifier device in which the output of an amplifier element is fed back via a first and a second negative feedback path to the two inputs. One negative feedback path includes a resistor and the other a voltage-controlled voltage source. An inverting differential amplifier is thus produced overall which has an improved noise behavior compared with an inverting differential amplifier connected up in a conventional manner. However, the resistor in the first negative feedback path still supplies a finite contribution to the total noise of the amplifier device. This is because the input of the amplifier device is connected up directly to the resistor.
Moreover, DE 40 24 166 C1 discloses an amplifier device having capacitive negative feedback which is likewise distinguished by a good noise behavior. This is because the capacitors used in this case also have very little noise. What is unfavorable, by contrast, is that the gain of the disclosed amplifier device having capacitive and negative feedback is greatly dependent on the frequency and on the load.
However, an amplifier device having a high bandwidth is favorable precisely when used in an ultrasonic apparatus, because of the requirements of being able to connect to different ultrasonic transducers, in particular those having center frequencies that differ from one another. The required bandwidth may in this case be of the order of magnitude of at least two decades.
Furthermore, the known amplifier devices often have a very high and occasionally also an undefined input impedance. The input impedance is typically at least 10
4
&OHgr;. This has an unfavorable effect with regard to power matching of the amplifier device to a source resistance of the ultrasonic transducer connected to the input. This source resistance is of the order of magnitude of a few 10 &OHgr;. If appropriate, the connected ultrasonic transducer may also be provided with a simple matching circuit which transforms the source resistance to a standard value of e.g. 200 &OHgr;. However, this value is significantly below the value which is customary for the input impedance of the known amplifier devices. Apart from the power transfer which is not optimum in that case, the high or undefined input impedance can additionally also lead to a deterioration in the noise behavior. Furthermore, standing wave effects can occur, leading to undesirable distortion.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide an amplifier which overcomes the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type in such a way that the gain of the amplifier is as independent of frequency as possible in a wide frequency range, and the amplifier has a defined real input impedance. Moreover, the intention is for the amplifier device to have as little noise as possible and to be readily integratable. In particular, the intention is also to eliminate the noise contribution of the negative feedback.
With the foregoing and other objects in view there is provided, in accordance with the invention, a low-noise, broadband amplifier, that includes a first broadband amplifier element having a first input, a second input, and a first output. A first negative feedback path is fed back from the first output to the first input, a second negative feedback path is fed back from the first output to the second input, and a controlled current source is disposed in the first negative feedback path.
In this case, the invention is based on the insight that, by means of a controlled current source in the first negative feedback path, it is possible both for the noise contribution of the second negative feedback path, required for setting the gain, to be reduced and for a real finite input impedance to be set. What has a particularly favorable effect in this case is that the setting of the real finite input impedance does not require an additional non-reactive resistor, the thermal noise of which would result in the noise behavior of the entire amplifier device also being impaired. Specifically, the controlled current source in the first negative feedback path has the effect that a virtual input impedance having a real and finite value forms at the input of the amplifier device. In particular, this does not require a resistive connection in parallel with the input of the amplifier device. The advantage of matching to the source resistance of a unit, for example an ultrasonic transducer, connected upstream of the amplifier device is thus accompanied by an improvement in the noise behavior.
Without any restriction to the general validity, the effect of the controlled current source in the first negative feedback path will now be explained for the case where the first amplifier element having negative feedback via the second negative feedback path is an operational amplifier connected up in a non-inverting fashion. The ideally infinite input impedance of the non-inverting operational amplifier can now be transferred by the controlled current source into an input impedance having a finite and real value. Specifically, the controlled current source converts the output voltage of the amplifier device into a current which appears, at the input of the amplifier device, as an input current which is proportional to the output voltage. On account of the proportional relationship—determined by the gain—between the input voltage and the output voltage, a proportional relationship between the input current and the input voltage thus also res
Greenberg Laurence A.
Locher Ralph E.
Mottola Steven J.
Siemens Aktiengesellschaft
Stemer Werner H.
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