JPS6010108Y2 - Wideband balanced amplifier with gain switching function - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a broadband balanced amplifier having a gain switching function, which is used as a vertical amplifier or horizontal amplifier of a cathode ray tube oscilloscope.

FIG. 1 is an example of a conventional wideband amplifier used in a vertical amplifier circuit of a wideband oscilloscope.

The amplifier circuit of an oscilloscope must have a cut-off frequency high enough to match the bandwidth of the oscilloscope.

For this reason, conventionally, a cascode-connected amplifier as shown in FIG. 1 has been used.

In Figure 1, 1 and 2 are input terminals for balanced signals, 3 and 4
is a balanced output terminal.

The common emitter transistors 20 and 21 are each
Together with the base-grounded transistors 22 and 23, a cascode amplifier is formed.

5 is a feedback resistor, and its ratio with the load resistor 10.11 is selected to be a low value of about 300 to 500 in order to obtain a gain.

In the conventional circuit shown in FIG. 1, switch 8 is provided for gain switching, and when switch 8 is closed, resistor 9
is connected in parallel with the feedback resistor 5, so the voltage gain is approximately doubled.

However, in this conventional circuit, (1) switching the gain to a higher value lowers the high-frequency cutoff frequency, (2) increasing the gain lowers the input impedance of the amplifier, and (3) switch 8 is a transistor. 20. If it is not placed very close to 21, the amount of feedback in the high frequency range will increase due to the influence of the lead wire inductance, which will further lower the cutoff frequency. There are some drawbacks.

Here, (1) will be explained in more detail using the circuit shown in FIG. 2 and the equivalent equinodal circuit shown in FIG. 3.

In Figures 2 and 3, rg is the signal line impedance, rI is the input impedance, RE is the feedback resistance, and rb'
is the base resistance, r8 is the emitter spread resistance, and 1α.

is the common base current amplification factor in direct current, β0 is the common emitter amplification factor in direct current, and ωt is the angular frequency above the frequency at which the current amplification factor becomes 1.

In the circuit of FIG. 3, the input impedance r! is rlβoxRE ・・・・・・・・・・・・
...(1) Also, the output current i.

The ratio of the input voltage Vl and the input voltage Vl is as follows.

S is multiple frequencies. Therefore, the value of io/v1 becomes 1/j2 of the value in the low range.

Frequency or cutoff frequency k.

From equation (3), it is assumed that RE is made smaller.

It can be seen that this decreases.

An oscilloscope that uses a circuit like the one shown in FIG. 1 as a vertical deflection amplifier has the disadvantage that when the gain is switched to improve deflection sensitivity, the cutoff frequency of the oscilloscope decreases.

Furthermore, as can be understood from equation (1), changing RE changes the input impedance, which sometimes makes the operation of the preceding stage amplifier circuit unstable.

For this reason, in the past, transistors with a high z-tooth were used to make the cutoff frequency of the gain switching amplifier circuit sufficiently higher than the cutoff frequency of the oscilloscope. so that it has almost no effect on

However, 70 high transistors are expensive.

Also, switching the gain allowed the cutoff frequency of the oscilloscope to change.

Furthermore, β is added to reduce the influence of changes in input impedance.

It used large transistors. The present invention solves the above-mentioned drawbacks of the conventional technology, and allows an amplifier with a high cutoff frequency to be constructed using easily available and inexpensive transistors, and even when the gain is switched using a switch located far from the transistor, the cutoff can be cut off. The present invention provides a broadband balanced amplifier having a gain switching function that does not change much in frequency or input impedance and can be easily integrated into an IC.

The present invention will be explained in detail below with reference to the drawings.

FIG. 4 shows one embodiment of the present invention.

In this example, the first. second transistor 20.21
Approximately l Q
Since a bias current of mA is flowing, the output impedance of each emitter is set to about several ohms.

Therefore, the third. The base of the fourth transistor 24.25 is the first point of low output impedance. Since it is connected to the emitter of the second transistor 20.21,
Ta.

Even if the transistor does not have a high current, the high cutoff frequency of its collector output current can be set high.

Also, transistors 20 and 24 and transistor 21
and 25 are bias resistors 6, 7, 13, 14, the first feedback resistor 5, and the second
The feedback resistor 12 is selected.

Furthermore, the switches 8a and 8b are interlocking switches.

Resistors 15, 16 are bias resistors for providing a bias current that flows through transistors 24, 25 when they are conductive.

Next, the operation will be explained.

(a) In the case of low gain state, switches 8a and 8b
is off and transistors 24 and 25 are also off.

The voltage gain is the ratio of resistors 5 and 10 (150Ω/1 in the case shown)
00Ω = 1.5) and the ratio of resistors 5 and 11 (150Ω/100Ω = 1.5 in the illustrated case).

(In case of high gain state, switch 8a.

8b is on, and transistors 24 and 25 are also on.

The voltage gain is approximately (ratio of resistors 5 and 10, or 1.5 in the case shown) and (ratio of resistors 12 and 10, or 1 in the case shown).
50Ω/100Ω=1.5) (1.5+ in the case shown)
1.5=3), so in case (a) it is almost twice as large.

Generally, the ratio of the two high and low voltage gains to be switched is n
(>1), the value of the second feedback resistor 12 is approximately 1/(n-1) of the value of the first feedback resistor 5.

Since transistors 24.25 are driven by the emitter of transistor 20.21, which is the point of low output impedance, the high cutoff frequency of this amplifier is high.

Therefore, even when switching from state (a) to state (b), the cutoff frequency does not decrease.

Also, there is little change in input impedance.

In order to take full advantage of the effects of the present invention, it is preferable to use a cascode connection as shown in FIG. 4 and to not provide high voltage gain to the transistors 20, 21, 24 and 25.

This is because if transistors 20, 21, 24゜25 have a voltage gain, the input capacitance seen from terminals 1 and 2 will change due to the Miller effect, and the output impedance of the preceding circuit connected to terminals 1 and 2 will change. This is because the cutoff frequency determined by the input capacitance at terminals 1 and 2 changes.

FIG. 5 shows an example of application of the present invention.

In this application, switches 8a, 8b are motional.

The gain of the switch 30 is changed depending on whether it is connected to the P side or the Q side when the switches 8a and 8b are on.

In this application example, the gain is switched in three stages.

The operation of this application example is as follows.

(a) When the gain is the smallest, switch 8a
+8b is on, and switch 30 is connected to the P side.

Transistors 24.25 are on and transistors 28.29 are also on.

Also, transistors 26 and 27 are off.

The voltage gain is (ratio of resistors 5 and 10) and (ratio of resistors 12 and 10)
It is determined by the difference between

(b) When the gain is intermediate, the switches 8a and 8b are off and the switch 30 remains switched to the P side.

Transistors 24 and 25 are turned off, and the voltage gain is determined by (ratio of resistors 5 and 10).

(C) When the gain is maximum, switches 8a and 8b are on and switch 30 is connected to Q.

Transistor 24.25 is on, transistor 26.27 is also on, and transistor 28.29 is off.

The voltage gain is (ratio of resistors 5 and 10) and (ratio of resistors 12 and 10)
It is determined by the sum of

As described above in detail, the broadband balanced amplifier of the present invention has the following advantages.

(1) Even if the voltage gain is changed, the cutoff frequency does not change much.

(2) Even if cheap transistors with low fr are used (1)
effect can be obtained.

(3) Even if the voltage gain is switched, the input impedance does not change.

(4) Easy to convert into IC.

(5) Even if the gain switching switch is far away, the cutoff frequency does not decrease.

Therefore, especially when used in a vertical amplifier of a wideband oscilloscope, which cannot achieve sufficient performance unless it is integrated into an IC and miniaturized, an inexpensive deflection sensitivity switching circuit with little change in cutoff frequency can be obtained.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an example of a conventional balanced amplifier, FIGS. 2 and 3 are isode circuits for explaining the operation and drawbacks of the conventional balanced amplifier, and FIG. 4 is a circuit diagram showing an example of the present invention. FIG. 5 is a circuit diagram showing an example of application of the present invention. 1.2... 1st. Second balanced input terminal, 5...
...First feedback resistor, 20, 21...First
．． 2nd transistor, 24.25...3rd.
Fourth transistor, 26, 27, 28, 29...
...Switching control transistor, 12...Second feedback resistor, 13°14, 8a, 8b, 15.
16"""control means, 3.4... 1st. 2nd
balanced output terminals, 22, 10... first combining means, 23, 11... second combining means.

Claims (1)

[Scope of utility model registration request] In a wideband amplifier having a gain switching function to take either of two desired voltage gains, the first. a second balanced input terminal and a respective base of the first . The first. a second transistor; When the ratio of the two voltage gains in which the base is connected to each emitter of the second transistor and the respective emitters are connected in series is set to n (>1), the value of the first feedback resistor is approximately 1/(n -1) forming a common emitter amplifier with a second feedback resistor of value . a fourth transistor; The fourth transistor's via or current is controlled to control the fourth transistor's via or current. a control means for turning on and off the fourth transistor; a second balanced output terminal; a first synthesizing means that synthesizes each collector current of the first transistor and the third transistor and supplies the sum to the first balanced output terminal; and a second synthesizing means which synthesizes each collector current of the fourth transistor and supplies it to the second balanced output terminal.