JPH02118893A - Logarithmic converting circuit - Google Patents

Abstract

PURPOSE:To obtain the title circuit, which is composed of the minimum number of elements and pins and executes a stable operation, by logarithmically changing the difference between an added result obtained by adding the potential difference between the respective Bs (base) and Es (emitter) of two transistors (Trs) and potential difference between the respective anodes and cathodes of two diodes for the difference of the voltages respectively applied to the respective Bs of other two Trs. CONSTITUTION:Voltage VB of a constant voltage source 18 is applied to the respective bases of transistors (Trs) 1 and 2, and the respective anodes of diodes 3 and 4 are connected to the respective emitters of the Trs 1 and 2. Further, the collectors of Trs 5 and 6 are connected to the respective cathodes of the diodes 3 and 4, and different voltages are applied to the respective bases of the Trs 5 and 6. The difference between an added result obtained by adding the potential difference between the respective base and the emitter of the Tr 1 and 2, and the potential difference between the anode and the cathode of the diode 3 and 4 logarithmically changes for the difference of the voltage applied to the respective base of the Tr 5 and 6. Namely, when voltage Vin is impressed to an input terminal 17 of this circuit, the variation is converted into a logarithm and appears at output voltage terminals 19 and 20.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a logarithmic conversion circuit that can be used in electrical circuits of electronic communication equipment such as televisions, VTRs, audio equipment, and satellite broadcast receivers.

Conventional technology Conventionally, in a logarithmic conversion circuit using a transistor, a relational expression between the base-emitter voltage vBE of the transistor and the emitter current IE, k: Boltzmann constant,
A circuit using τ - Absolute Temperature 9V is used, and a circuit using an operational amplifier is often used as a practical circuit. Conventional 15
IIj gold shown in Figure 2. FIG. 2 shows all practical examples of a typical logarithmic conversion circuit using an operational amplifier.

21 and 22 are operational amplifiers, 23 and 24 are transistors for obtaining logarithmic characteristics, and transistors with uniform characteristics are used. 25, 26, 2a, 28 are resistors that determine the customization of the output voltage, 29 is an input terminal, 3° is an output terminal, 31 is a reference voltage source for determining the output voltage, 32 and 33 are for preventing oscillation. It is a condesa.

Problems that the invention aims to solve In the conventional example, the output voltage vout is expressed by the following equation.

Here, R1, R2, R5, and R6 are resistances of 25, 26° and 27.28 degrees, respectively, vi is the input cuff 1NB, and vre is
f” is the reference voltage.

In this case, it is necessary to use transistors 23 and 24 shown in FIG. 2 that have the same characteristics. When converting a conventional circuit into an IC, the number of elements increases because it incorporates two differential input high-gain (several tens of dB) amplifiers that function as one or two operational amplifiers.(2) Therefore, consideration must be given to preventing oscillation.

(1 (2) requires a capacitor to prevent oscillation, and if it is built-in, the area will increase and the cost will increase; if it is externally mounted, four pins are required to attach the capacitor, etc.) Think about the problem.

In view of the above-mentioned problems, the present invention provides means for realizing a circuit with stable operation using a small number of elements and a small number of pins when a logarithmic conversion circuit is built into an IC.

Means for Solving the Problems In order to solve the above problems, the logarithmic conversion circuit of the present invention has the following features: Applying equal voltages to the bases of each of the second transistors and applying equal voltages to the emitters of each of the first and second transistors.
, the anodes of each of the second diodes are connected;
The third diodes are connected to the cathodes of each of the second diodes. mF the collector of the fourth transistor; By applying different voltages to the bases of each of the fourth transistors, there is a potential difference between the bases and emitters of each of the first and second transistors and the first and second transistors. The difference between the potential difference between the anode and the quad of the second diode is the difference between the third and fourth diode.
The structure takes full advantage of the fact that the voltage varies logarithmically with respect to the difference in voltage applied to the bases of each transistor.

Effects The present invention has the above-described configuration, (1) it is possible to reduce the area occupied by the grooves with a small number of elements. (2) Stability is good because no negative feedback circuit is used, and no capacitor is required to prevent oscillation. It has the following advantages, and is particularly effective when integrated into an IC.

Embodiments In the following, we will clarify the details of the pair-to-converter circuit of this research without referring to all the drawings.

FIG. 1 shows a circuit diagram of a logarithmic conversion circuit in an embodiment of the present investigation. In FIG. 1, 1 and 2 are transistors 3, 4 are diodes, 6.6 are transistors, 7.8 are diodes, and 9R1, 10R2, 11R3, 12R4.

13Rs T 14Re , 1tsR-r are resistors, 16 is a reference voltage source, 17 is an input voltage terminal vIN-18
is the direct voltage source at the base of transistor 1゜2, 19,2
0 is the output direct voltage terminal ■. , v2.

What about the bases of transistors 1 and 2? Voltage VB is supplied from l[j pressure source 18. The emitters of transistors 1, 2 are each connected through a diode 3.4 to the respective collector of a transistor 6.6. The base of the transistor 5 is connected to a reference voltage source 16 (potential vref) through a resistor 9, and the base of the transistor 6 is connected to a reference voltage source 16 (?[ref) through a resistor 10.Transistor 6, diode 7, resistor 11.12 constitutes a current mirror, which includes a transistor 6, a diode 8, a resistor 13.
14 buys all power 1/71-mirror. transistor 6
The base of the resistor is 16 gold 1ffi and the input voltage terminal 17
It leads to

The transistor 6.6 and the diode 7.8 have the same characteristics, R1:R2, R3:=R4, R6=R6.

At this time, assuming that the DC current amplification factor of transistor 5.6 is sufficiently large, (vD1 = voltage across diode 3, vBE3 = base-emitter voltage of transistor 5) ("D2 = ta 4 ot' 4 (7) Press both ends*
VBB; 4 = ) -y Between the base and emitter of sensor 6 [E) E3 = E1. Engineering E4 = Engineering 2. R1+
Since R3 = R2 + R4, vD1 = vD2, and for transistor 1 3.4, E1 = IC3 - E3 IE2 = C4 - IE4. If the characteristics of the diodes 3 and 4 are made the same, the following will be obtained.

v1=”B-”BEl-vDl(”BEl=) rough sinuli 11) Base-emitter voltage) v2-VB-vBE2-vD2 (vBE2=base-emitter voltage of transistor 2) vBE1=vBE2.vD1= From vD2, that is, when there is no signal, the potential of the output voltage terminal 19.20 v1.v2
are equal.

Next, consider the case where the voltage vin is applied to the input voltage terminal 17 of this circuit. At this time, IE4 is set to 'pl< and work s1
: Saturation current of transistor 1.2 s2: Diode 3
．． Saturation current of 4 "E4+Δ■F4 ・・・・・・(4) ;co IE2 k 1f2・vBx2 'kV
BE et al. I vD2 k vD6 If ev2 is v2', then v2 = VB - vB12 - vD6 ・(@ Therefore, the output voltage terminal 19 at this time. is converted into logarithm and output voltage terminal Vout1
, Vout2. In other words, a logarithmic conversion circuit can be constructed.

Effects of the Invention With the above configuration, the present invention can be constructed with fewer elements and occupy less space. ■Since it does not use a negative feedback circuit, it has good stability and does not require a capacitor to prevent oscillation. These effects are particularly effective when integrated into an IC.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a logarithmic conversion circuit according to an embodiment of the present invention, and FIG. 2 is a circuit diagram of a logarithmic conversion circuit according to a conventional example. 1.2, 5.6...Transistor, 3, 4
, 7°8...Diode, 9, 10, 11, 1
2,13°14.16...Resistance, 18.18.
... Constant voltage power supply, 17 ... Input voltage terminal, 19, 20 ... Output voltage terminal. That is, the input voltage V, by this circuit. I'm disappointed in the fact that I applied all the power.

Claims (1)

[Claims] Applying an equal voltage to the bases of each of the first and second transistors, connecting the anodes of each of the first and second diodes to the emitters of each of the first and second transistors, and The collectors of the third and fourth transistors are connected to the cathodes of the diodes of the third and fourth transistors.
, a potential difference between the base and emitter of each of the first and second transistors and a potential difference between the anode and cathode of the first and second diodes, which are caused by applying different voltages to the bases of each of the fourth transistors. 2. A logarithmic conversion circuit characterized in that the difference between the voltages added to the bases of the third and fourth transistors varies logarithmically with respect to the difference between the voltages applied to the bases of the third and fourth transistors.