Analog switch circuit

Details Analog switch circuit Analog switch circuit

2000-11-02

2003-02-04

Cunningham, Terry D. (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Specific signal discriminating without subsequent control

By amplitude

C327S067000, C327S563000, C330S254000

Reexamination Certificate

active

06515518

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an analog switch circuit. The circuit has a plurality of analog differential signal inputs. Therein, one of signal inputs is selected.
The invention is particularly concerned with a wideband switch for switching differential input signals of DC to several GHz. The switch has low cross-talk characteristics isolated from non-selected signal channels.
2. Description of the Prior Art
A conventional oscilloscope has a switching circuit to select an input signal channel from a plurality of analog differential input signal channels.
Prior Art 1 of a conventional analog switch circuit is shown in FIG.
1
.
In
FIGS. 1
,
21
A and
22
A are the first channel differential signal input terminals, which are respectively connected to bases of transistors
1
A and
2
A. In like manner,
21
B and
22
B are the second channel differential signal input terminals, which are respectively connected to bases of transistors
1
B and
2
B.
Emitters of transistors
1
A and
2
A are connected with each other via resistors
11
A and
12
A. The junction point of resistors
11
A and
12
A is connected with a terminal of a constant current source
16
A via a switch
15
A. Another terminal of the source
16
A is connected to a power source V
EE
.
Emitters of transistors
1
B and
2
B are connected with each other via resistors
11
B and
12
B. The junction point of resistors
11
B and
12
B is connected with a terminal of a constant current source
16
B via a switch
15
B. Another terminal of the source
16
B is connected to the power source V
EE
.
Collectors of transistors
1
A and
1
B are connected to a power source V
CC
via a load resistor
33
. Collectors of transistors
2
A and
2
B are connected to the power source V
CC
via a load resistor
34
.
A pair of transistors
1
A and
2
A forms a differential amplifier. Another pair of transistors
1
B and
2
B forms another differential amplifier.
When the switch
15
A is on and the switch
15
B is off, the first pair of transistors
1
A and
2
A amplifies the first differential input signal between input terminals
21
A and
22
A to obtain a differential output between differential signal output terminals
37
and
38
. The second pair of transistors
1
B and
2
B does not amplify the second differential input signal between input terminals
21
B and
22
B, because of no collector currents of transistors
1
B and
2
B.
When the switch
15
A is off and the switch
15
B is on, reversely, the second pair of transistors
1
B and
2
B amplifies the second differential input signal between input terminals
21
B and
22
B to obtain a differential output between differential signal output terminals
37
and
38
. The first pair of transistors
1
A and
2
A does not amplify the first differential input signal between input terminals
21
A and
22
A, because of no collector currents of transistors
1
A and
2
A.
Therefore, by means of on-off switching operation of switches
15
A and
15
B, the analog switch circuit shown in
FIG.1
can selectively amplify a signal of two differential input signals. The first signal between input terminals
21
A and
22
A, or the second signal between input terminals
21
B and
22
B, is selectable.
In
FIG. 1
, two differential amplifiers of two pairs of transistors
1
A,
2
A and
1
B,
2
B are shown. Many differential amplifiers with switches
15
s
are usable, too. When only one of switches
15
s
is on and the other switches
15
s
are off, the only one differential signal is amplified to obtain the output between terminals
37
and
38
. However, the other differential input signals are not amplified.
The analog switch circuit shown in
FIG. 1
has the disadvantage of cross-talk, because of base-to-collector capacitances C
bc
s.
In spite of no collector current, high frequency ingredients of the input signals of bases leak out to collectors via the capacitances C
bc
s.
In
FIG. 2
, there are shown a base-to-collector capacitance C
bc
, a base-to-emitter capacitance C
be
and a collector-to-emitter capacitance C
ce
.
In
FIG. 3
, there is shown the base-to-collector capacitance C
bc
depending on the collector-to-base voltage.
The base-to-emitter capacitance C
be
and the collector-to-emitter capacitance C
ce
, which are not shown in
FIG. 3
, have the same characteristics as that of the base-to-collector capacitance C
bc
.
Prior Art
2
is shown in Japanese Provisional Publication No. 10-285006. Therein, an analog switch circuit is disclosed. The circuit employs means to leak high frequency ingredients for reducing the cross-talk.
In
FIG. 4
, the circuit of the prior art
2
is shown. The first differential signal input terminals
21
A and
22
A are respectively connected to bases of transistors
1
A and
2
A.
The second differential signal input terminals
21
B and
22
B are respectively connected to bases of transistors
1
B and
2
B.
Emitters of the transistors
1
A and
2
A are connected to each other via resistors
11
A and
12
A. The junction point of resistors
11
A and
12
A is connected with a terminal of a constant current source
16
A via a switch
15
A. Another terminal of the source
16
A is connected to a power source V
EE
.
In like manner, emitters of the transistors
1
B and
2
B are connected to each other via resistors
11
B and
12
B. The junction point of resistors
11
B and
12
B is connected with a terminal of a constant current source
16
B via a switch
15
B. Another terminal of the source
16
B is connected to.the power source V
EE
.
Transistors
7
A,
8
A,
7
B and
8
B are employed. In each of them, the emitter is connected with the base. The base of the transistor
7
A is connected with that of
1
A. In like manner, the base of
8
A with
2
A,
7
B with
1
B and
8
B with
2
B.
The collector of the transistor
7
A is connected with that of
2
A. In like manner, the collector of
8
A with
1
A,
7
B with
2
B and
8
B with
1
B.
Transistors
1
A and
2
A form a differential amplifier with means to leak high frequency ingredients for reducing the cross-talk. Transistors
7
A and
8
A operate as the leak means.
In like manner, transistors
1
B and
2
B form a differential amplifier with means to leak high frequency ingredients for reducing the cross-talk. Transistors
7
B and
8
B operate as the leak means.
When the switch
15
A is on and the switch
15
B is off, the first differential amplifier of transistors
1
A and
2
A amplifies the first differential input signal between input terminals
21
A and
22
A to obtain a differential output between differential signal output terminals
37
and
38
.
The second differential amplifier of transistors
1
B and
2
B does not amplify the second differential input signal between the second differential input terminals
21
B and
22
B, because of no collector currents of transistors
1
B and
2
B.
However, a part of high frequency ingredients of the second differential input terminals
21
B and
22
B appears at collectors of transistors
1
B and
2
B by passing through base-to-collector capacitances C
bc
s of transistors
1
B and
2
B.
Transistors
7
B and
8
B, which have base-to-collector capacitances C
bc
s, leak a part of high frequency ingredients to collectors
2
B and
1
B in reverse phase respectively. Therefore, if the both leaks of transistors
1
B and
8
B are equal in their amplitude, the leaks can be canceled, because of their phase reverse to each other. In like manner, the leaks of transistors
2
B and
7
B can be canceled, because of their phase reverse to each other.
Actually, collector-to-base voltages of transistors
1
B and
2
B are not equal, and their base-to-collector capacitances are not same in value. Their capacitances vary in value depending on the collector-to-base voltages which are varied by differential input signal between input terminals
21
B and
22
B. Therefore, the cancellation by using leaks is not satisfied. The dispersion of base-to-collector capacitances is one of impedimental factors of the cancellation.
SUMMARY OF THE INVENTION
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