Peak clipper with an expanded linear characteristic region for video signals

A peak clipper according to this invention contains a differential circuit composed of a first and second p-n-p transistors, the first transistor's base terminal receiving the input signal and the second transistor's base terminal receiving a reference voltage for determining a clipping voltage to clip the input signal at a specified voltage level Connected to the collector of the second transistor is a current mirror circuit composed of a third and fourth n-p-n transistors. The current path of the current mirror circuit is connected to the base of a fifth n-p-n transistor, whose emitter is connected to the base of the second transistor. The reference voltage is changed so as to boost the current driving capability of the second transistor at the beginning of the switching operation at the differential circuit.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a peak clipper for use in a pre-stage of the 
modulating circuit that amplitude-modulates a video signal. 
2. Description of the Related Art 
A peak clipper is used in the RF modulator built in a VCR (Video Cassette 
Recorder), for example. The RF modulator converts the processed signal in 
the VCR into a radio frequency (RF) suitable for television. The peak 
clipper prevents signals other than the specified video signal, such as 
noise, from entering the video signal modulating circuit in the RF 
modulator. 
In the clipper circuit operation, it is important that variations in the 
video signal lower than the clipping level are transmitted to the next 
stage, maintaining the linear characteristics. Since most peak clippers 
simply make use of the switching operation at a differential transistor 
pair, the approach of the clipping level makes the clipping operation 
slower, worsening the linear characteristics. That is, gradual start of 
clipping at a voltage below the clipping level makes the linear region 
narrower. As a result, a signal of high luminance or large amplitude can 
enter the linearity-collapsed region near the clipping level, which 
distorts the video signal. 
SUMMARY OF THE INVENTION 
Accordingly, the subject of the present invention is to provide a peak 
clipper causing the voltage to make a sharp change in the vicinity of the 
clipping point. 
The foregoing object is accomplished by a peak clipper, comprising: a 
differential circuit composed of a first and second transistors of a first 
polarity, the base terminal of the first transistor receiving the input 
signal and the base terminal of the second transistor receiving a 
reference voltage for determining a clipping voltage to clip the input 
signal at a specified voltage level a first voltage supply circuit for 
supplying as the reference voltage an offset voltage after the switching 
operation at the differential circuit that clips the input signal; a 
second voltage supply circuit for supplying as the reference voltage a 
potential difference proportional to the collector current of the second 
transistor in such a manner as to apply positive feedback to the base 
terminal of the second transistor at the beginning of the switching 
operation at the differential circuit; a positive-phase output terminal 
connected to the common emitter of the first and second transistors; and a 
negative-phase output terminal for receiving variations in the potential 
difference from the first and second voltage supply circuit. 
With this arrangement, the second voltage supply circuit, which applies a 
potential difference proportional to the second transistor's collector 
current to the second transistor's base terminal in a positive feedback 
manner, not only promotes the switching operation at the differential 
circuit, but also lowers the negative-phase output voltage. This approach 
compensates for slow variations in the differential voltage between the 
positive-phase and negative-phase outputs. 
Additional objects and advantages of the invention will be set forth in the 
description which follows, and in part will be obvious from the 
description, or may be learned by practice of the invention. The objects 
and advantages of the invention may be realized and obtained by means of 
the instrumentalities and combinations particularly pointed out in the 
appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the accompanying drawings, the present invention will be 
explained by means of an embodiment. 
FIG. 1 is a circuit diagram for a peak clipper according to an embodiment 
of the present invention. It is assumed that the base-emitter voltage 
V.sub.BE is the same for all of n-p-n and p-n-p transistors constituting 
the FIG. 1 circuit and that their base currents are all negligible. FIG. 4 
shows a video modulating circuit in which the peak clipper circuit of FIG. 
1 may be applied. 
The input signal V.sub.in is supplied to the base of an n-p-n transistor 
Q1. The transistor Q1 has the collector connected to the supply voltage 
V.sub.CC and the emitter connected to the ground voltage GND via a 
constant current source I4 as well as to the base of a p-n-p transistor 
Q2. The transistor Q2 has the collector connected to the GND and the 
emitter connected to the emitter of a p-n-p transistor Q3. Both emitters 
of the transistors Q2 and Q3 are connected to V.sub.CC via a constant 
current source I2 as well as to the output terminal for the positive-phase 
output V.sub.OUT (+). The collector of the transistor Q3 is connected to 
the base and collector of an n-p-n transistor Q4, whose emitter is 
connected to the GND. 
The base of the transistor Q3 is connected to the ground voltage GND via a 
constant current source I1 as well as to one end of a resistor R1. The 
other end of the resistor R1 is connected to both the base of a p-n-p 
transistor Q5 and the emitter of an n-p-n transistor Q6. The transistor Q6 
has the collector connected to the supply voltage V.sub.CC and the base 
connected to both. one end of a resistor R2 and the collector of an n-p-n 
transistor Q7. The other end of the resistor R2 is the input terminal for 
a reference voltage V.sub.ref. The transistor Q7 has the base connected to 
the base of the transistor Q4 and the emitter connected to the GND. 
The collector of the transistor Q5, whose base is connected to the other 
end of the resistor R1, is connected to the GND. The emitter of the 
transistor Q5 is connected to V.sub.CC via a constant current source I3 as 
well as to the output terminal for the negative-phase output V.sub.out 
(-). 
The operation of the circuit thus constructed will now be explained, 
referring to the characteristic curves in FIGS. 2 and 3. Here, the output 
value of the circuit is the differential voltage between V.sub.out (+) and 
V.sub.out (-) in FIG. 2 and shown in FIG. 3 as V.sub.out (+)-V.sub.out 
(-). The circuit is constructed so that with no clipping operation, the 
extension may have an inclination meeting V.sub.out (+)=V.sub.out (-) when 
V.sub.in =V.sub.ref as indicated by a broken line in FIG. 3. 
In the circuit with the above arrangement, the clipping level is set on the 
basis of a voltage drop of R1.multidot.I1. Specifically, when the voltage 
value of the input signal V.sub.in exceeds the voltage value of v.sub.ref 
-R1.multidot.I1, a clipping operation will take place, with the result 
that the output value of V.sub.out (+)-V.sub.out (-) is the voltage of 
-R1.multidot.I1 and remains unchanged. 
When V.sub.in &lt;&lt;(V.sub.ref -R1.multidot.I1), the base of the transistor Q2, 
which together with the transistor Q3 composes the differential circuit, 
is impressed by a voltage of V.sub.in -V.sub.BE, causing the transistor Q2 
to be on. As a result, there is no current flow in the current mirror 
circuit composed of the transistors Q4 and Q7, creating no voltage drop 
across the resistor R2. Thus, the conditions of V.sub.out (+)=V.sub.in and 
V.sub.out (-)=V.sub.ref are established, so that the output varies 
according to the input. 
When V.sub.in &gt;&gt;(V.sub.ref -R1.multidot.I1), the transistor Q2 will be off 
and transistor Q3 be on, which allows current I2 to flow through the 
resistor R2 via the current mirror composed of the transistors Q4 and Q7. 
In this situation, clipping operation takes place to produce V.sub.out (+) 
and V.sub.out (-), each given as: 
EQU V.sub.out (+)=V.sub.ref -R2.multidot.I2-R1.multidot.I1 (1) 
EQU V.sub.out (-)=V.sub.ref -R2.multidot.I2 (2) 
Thus, these outputs are fixed and the following equation is obtained: 
EQU V.sub.out (+)-V.sub.out (-)=-R1.multidot.I1 (3) 
In the vicinity of V.sub.in =V.sub.ref -R1.multidot.I1, the differential 
circuit made up of the transistors Q2 and Q3 performs switching operation. 
The present invention is characterized in that voltage supply means is 
employed to apply voltage feedback the base of the transistor Q3, where 
the reference voltage is supplied to the differential circuit. In this 
preferred embodiment, the voltage supply means includes the transistor Q6 
and its associated circuitry including the resistors R1 and the resistor 
R2, and the current mirror circuit composed of the transistors Q4 and Q7. 
FIG. 2 shows voltage variations in V.sub.out (+) and V.sub.out (-). As the 
input signal V.sub.in rises close to V.sub.ref -R1.multidot.I1, the 
transistor Q3 starts to turn on, permitting a current equivalent to the 
collector current of the transistor Q3 to flow through the resistor R2. 
This lowers the emitter voltage of the transistor Q6, causing the base 
voltage of the transistor Q3 to fall. This decreased base voltage promotes 
the switching operation at the differential circuit composed of the 
transistors Q2 and Q3 and at the same time, lowers the base voltage of the 
transistor Q5 to decrease the output value V.sub.out (-). Thus, setting 
the resistor R2 and current I2 to suitable values enables variations in 
V.sub.out (+) and V.sub.out (-) to cancel out each other, narrowing the 
region where the output changes in an unideal manner, as shown in FIG. 3. 
The transistor Q6 acts as a voltage buffer; the transistor Q1 performs 
level shift to compensate for the transistor Q6's V.sub.BE ; and the 
transistor Q5 carries out level shift to compensate for the V.sub.BE of 
the transistors Q2 or Q3. It should be noted that these transistors are 
not necessarily required. 
With the above embodiment, as shown in FIG. 3, slow variations in the 
differential voltage output at the clipping point 32 is compensated for, 
which provides a sharp voltage change near the clipping point 32. This 
narrows the region 31 where the output varies in an unideal manner, 
thereby broadening the linear region. As a result of this, the distortion 
of a large-amplitude video signal is improved. 
Additional advantages and modifications will readily occur to those skilled 
in the art. Therefore, the invention in its broader aspects is not limited 
to the specific details, and representative devices, shown and described 
herein. Accordingly, various modifications may be without departing from 
the spirit or scope of the general inventive concept as defined by the 
appended claims and their equivalents.