WO1998010874A1

WO1998010874A1 - Amplifying circuit for reading a capacitive input

Info

Publication number: WO1998010874A1
Application number: PCT/DE1996/001759
Authority: WO
Inventors: Michael Schmidt; Egbert Spiegel
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 1996-09-12
Filing date: 1996-09-12
Publication date: 1998-03-19

Abstract

The invention concerns an amplifying circuit for reading a capacitive input (100), having a first field effect transistor (102), the source electrode of which is connected to the capacitive input (100). A polarization current source IS is connected to the drain electrode of the first field effect transistor (102). The drain electrode of the first field effect transistor (102) forms an output node. The source electrode of the first field effect transistor (102) is connected to a regulated source of electricity. The gate electrode of the first field effect transistor (102) is switched to a reference potential according to a shift signal. In such a circuit the entire polarization current IBias of the transistor is available as charging current for the capacity of the capacitive input.

Description

Amplifier circuit for reading out a capacitive input

description

The present invention relates to an amplifier circuit for reading out a capacitive input, and in particular to such an amplifier circuit which can advantageously be used in a device which can be switched over between transmitting and receiving operation, for example an ultrasound system.

In the field of ultrasound diagnostics, it is common to use an ultrasound element as a transmitter and receiver at the same time. For this purpose it must be ensured that the reflected signals can be selected by switching on the readout electronics after a very short settling time in the range of a few 100 nanoseconds. In many ultrasound systems, the time until the reflected signals arrive at the receiving electronics, i.e. the amplifier circuit for reading out the received signals, large, in the range of a few microseconds, since this time span is extended by a coupling-in link. This coupling section serves to acoustically adapt the ultrasound elements to the medium to be examined.

With diagnostic systems for "in vivo" examinations of small vessels or with a direct coupling to a medium to be examined, it is possible that there is no coupling path. Therefore, the first reflections from vessel walls reach the ultrasonic transducer in the submicrosecond range.

State of the art for integrated miniaturized ultrasound readout amplifiers is the CMOS amplifier shown in FIG. 3. The ultrasound signal source 10 formed by an ultrasound transducer element is in FIG. 3 by a current source Iτ _j g and a capacitance connected in parallel therewith Modeled C _T JS. The ultrasound transducer element thus represents a capacitive source. The output of this ultrasound signal source is connected to the gate electrode of a field effect transistor. A resistor R _f , which is used to set the operating point of the amplifier, is connected between the gate electrode and the drain electrode of the transistor 12. This resistor, together with the capacitance CTJS, determines a time constant T which determines the settling time of the amplifier. The drain electrode of transistor 12 is also connected to a bias current source, which is modeled by a current source I _b i _.as . An output node 14 of the ultrasound readout amplifier is also connected to the drain electrode of transistor 12.

In the case of ultrasonic read-out amplifiers of the type shown in FIG. 3, the settling time can be set by setting the bias current I _b ias. A high bias current T.biaε is required to enable a sufficiently rapid settling. However, a high bias current results in a negative increase in power loss. In the known ultrasound readout amplifier, the bias current is further switched off in the transmission mode, ie when the ultrasound element is pulsed to generate ultrasound waves, in order to reduce the total power loss. The amplifier is only active in the receive mode, ie when receiving the reflected acoustic waves, with an ultrasonic transducer serving as a sensor. For example, in the case of "in vivo" systems, the available power loss is often limited, since the surroundings must not be heated. This problem of the limited available power loss arises more and more when a plurality of ultrasound devices are simultaneously excited and read out to focus an ultrasound beam. In this case there is only a limited power loss available per amplifier. Therefore, the bias current cannot be increased arbitrarily in known ultrasonic readout amplifiers, since the permitted power loss would then be exceeded. The object of the present invention is to provide an amplifier circuit for reading out a capacitive input and a method for operating the amplifier circuit, which enable a very short switchover time from transmission to reception mode with a very low power loss.

This object is achieved by an amplifier circuit according to claim 1.

The present invention provides an amplifier circuit for reading out a capacitive input with the following features: a first field effect transistor, the source electrode of which is connected to the capacitive input, a pre-current source which is connected to the drain electrode of the first field effect transistor, an output node, which is connected to the drain electrode of the first field effect transistor, and a regulated current source which is connected to the source electrode of the first field effect transistor, the gate electrode of the first field effect transistor being alternately connected to a reference potential.

By coupling the capacitive element, for example the ultrasound transducer, to the source terminal of the transistor, the entire bias current of the bias current source is available as a charging current for the capacitive input, for example the capacitance of the ultrasound element. According to an advantage of the present invention, the output voltage at the drain electrode of the field effect transistor can be kept at the operating point by regulating the current source, which is connected to the source electrode of the field effect transistor.

Further developments of the present invention are defined in the dependent claims.

Preferred embodiments of the present invention are explained in more detail below with reference to the accompanying drawings. Show it:

1 shows a schematic diagram of an amplifier circuit according to the invention;

2 shows a more detailed circuit diagram of an amplifier circuit according to the invention; and

Fig. 3 is a circuit diagram of a known amplifier circuit.

The present invention is explained in more detail below with reference to an ultrasound readout amplifier. However, it is obvious that the present invention is not limited to an amplifier for reading out ultrasonic elements, but can be used for reading out any capacitive input.

Fig. 1 shows a rough circuit diagram of a first embodiment of the present invention. An ultrasound element 100 is modeled as an ultrasound signal source by a current source I _us and a capacitance C _us . The output of the ultrasound element 100, ie the capacitive input, is connected to the source electrode of a transistor 102. The drain of field effect transistor 102 is connected to a bias current source Ifc> ias. A controlled current source, Ig, is connected between the source electrode of the field effect transistor 102 and a reference potential, for example ground.

The gate electrode of the field-effect transistor 102 is connected in terms of AC voltage to a fixed reference potential, for example signal ground. Because of this interconnection of the gate electrode, the circuit can be referred to as a "common gate stage".

1 is a further development of the present Shown invention, which further comprises a bias current controller (bias controller) 104. The bias current controller has connections which are connected to the drain electrode and the gate electrode of the field effect transistor 102, and furthermore a connection which is connected to a fixed reference potential, for example the signal ground. Another connection of the bias current regulator 104 is connected to the regulated current source Ig for controlling the same. Low-frequency voltage changes at the drain electrode of the field-effect transistor 102 can be suppressed by means of the bias current regulator 104 by regulating the current generated by the regulated current source Ig, as a result of which the amplifier can be kept at a desired operating point.

An example of such a control circuit 104 is shown in more detail in FIG. 2. In Fig. 2, elements corresponding to those in Fig. 1 are given the same reference numerals as those in Fig. 1. The regulated current source Ig is realized in the circuit shown in FIG. 2 by a second field effect transistor 200. The drain electrode of the second transistor 200 is connected to the source electrode of the first transistor 102. The source electrode of the second transistor 200 is connected to a fixed reference potential, for example the signal ground.

The bias current regulator 104 has transistors 202, 204 and 206, an active resistor 208 and a capacitance 210.

A field effect transistor is used as the active resistor 208, the drain electrode of which is connected to the drain electrode of the first field effect transistor 102, and the source electrode of which is connected to the gate electrode of the field effect transistor 202. The drain electrode of the field effect transistor 202 is connected to a constant potential, in the preferred embodiment preferably to the positive supply voltage. The source electrode of the field effect transistor 202 is connected to the drain electrode of the field effect transistor 204. The drain electrical de and the gate electrode of field effect transistor 204 are connected such that transistor 204 acts as a diode. The source of transistor 204 is connected to the drain of transistor 206. The drain electrode and the gate electrode of transistor 206 are connected such that transistor 206 also acts as a diode. The source electrode of transistor 206 is connected to a fixed reference potential, for example the signal ground. The gate electrode of transistor 206 is also connected to the gate electrode of second field effect transistor 200. Furthermore, the gate electrode of transistor 204 is connected to the gate electrode of first field effect transistor 102. The connection node of the source electrode of the field-effect transistor 208 acting as an active resistor and the gate electrode of the field-effect transistor 202 is connected via the capacitance 210 to the fixed reference potential, for example the signal ground. Furthermore, the gate electrode of transistor 208, which acts as an active resistor, is connected to the fixed reference potential, for example the signal ground.

The output of the amplifier stage shown in FIG. 2 is the drain node 220 of the first field effect transistor 102. The drain potential of the amplifying transistor 102 is maintained at the desired operating point by the regulated current source which is formed by the transistor 200 . The regulation takes place by coupling the transistors 202, 204 and 206 via the transistor 208 and the capacitance 210 to the output of the stage. The desired working point is thus set. Low frequency fluctuations at the input, i.e. at the capacitive input are suppressed. In contrast, high-frequency input signals are amplified by the controller's RC coupling, which is formed by the active resistor 208 and the capacitor 210. Overall, the stage shown has a high-pass characteristic, which is determined by the time constant of the controller.

The following is the control function of the bias current regulator 104 explained in more detail. A constant voltage is first assumed at the gate of the first field effect transistor 102. A voltage increase at the input, ie at the source electrode of transistor 102, appears increasingly at the drain electrode of transistor 102. If this voltage change at the input is low-frequency, this represents an undesirable shift in the operating point. This should be suppressed by the bias current regulator 104.

When the potential at the source of first field effect transistor 102 increases, the current flowing through transistor 102 decreases, whereupon the potential at node 220 increases because the bias current source Ißias continues to operate. Due to the increased potential at node 220, the potential present via active resistor 208 at the gate electrode of transistor 202 increases. This leads to a current flow through transistor 202 and further through transistors 204 and 206 connected as diodes. This increases the potential at the gate of transistor 200. This causes transistor 200 to conduct a higher current, whereupon the potential at the drain -Electrode of transistor 200 and thus at the source electrode of the first field-effect transistor 102 drops. As a result, the potential at point 220 is kept at the operating point of the amplifier by means of the bias current regulator 104 and the regulated current source 200.

For high-frequency signals, the RC coupling, which is formed by the active resistor 208 and the capacitor 210, switches this gate to the signal ground between the node 220 and the gate of the transistor 202. Therefore, the control described does not affect high-frequency signals.

The present invention thus provides an amplifier circuit for reading out a capacitive input, for example for reading out an ultrasound transducer element, in which the entire bias current I-ßi _{as of} the amplification transistor 102 is used as the charging current for the capacitance of the ultrasound elements is available. This improves the transient response of the amplifier when switching between transmit and receive operations, for example in an ultrasound system. Optionally, the amplifier circuit according to the invention can furthermore have a bias current regulator, which enables the potential of the output of the amplifier circuit to be kept at the operating point regardless of low-frequency potential changes at the input. This also improves the transient response of the amplifier when switching.

Claims

claims

1. Amplifier circuit for reading out a capacitive input (100) with the following features:

a first field effect transistor (102), the source electrode of which is connected to the capacitive input (100);

a pre-current source (I _ß ias) '_" ^^ ^em ^ ^αer drain electrode of the first field effect transistor (102) is connected;

an output node (220) connected to the drain of the first field effect transistor (102); and

a regulated current source (Ig; 200), which is connected to the source electrode of the first field effect transistor,

wherein the gate electrode of the first field effect transistor (102) is alternately connected to a reference potential.

2. The amplifier circuit of claim 1, further comprising a bias current control circuit (104) to control the regulated current source (I _s ; 200) such that the potential of the drain of the first field effect transistor (102) is applied regardless of low frequency voltage changes the source electrode of the first field effect transistor (102) remains constant.

3. The amplifier circuit according to claim 2, wherein the regulated current source is a second field effect transistor (200), the gate electrode of which is connected to the bias current control circuit (104), the drain elec- trode is connected to the source electrode of the first field effect transistor (102), and the source electrode of which is connected to a reference potential.