Oscilloscope Preamplifier

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A little about op amps and video amps by Frank C> Pugh

This article details the basic theory of operation of a video amplifier and the construction of a practical video amplifier project. I built this project to expand the sensitivity of an older oscilloscope. Before 1 begin a discussion on "video amplifier theory/' a short review of operational amplifiers {op amps) is useful.

Basic Amplifiers

An amplifier is an electronic circuit containing Bipolar Junction Transistor (BJT) and/or Field Effect Transistor (PET) devices, These types of amplifier circuits usually come packaged as FCs. These IC amplifiers generally provide voltage or current gain. They may also provide power gain or allow for desired impedance matching.

Amplifiers have many classifications. There are low-frequency amplifiers, audio amplifiers, ultrasonic amplifiers, radio frequency (RFj amplifiers, wideband amplifiers. op amps, and video amplifiers, each type operating within a prescribed frequency range or in a predetermined fashion. Op Amps

An operational amplifier is a highly sophisticated linear integrated circuit, a direct current amplifier demonstrating high gain, high input impedance, and low output impedance. Originally the term referred to high gain, high performance, vacuum tube direct current amplifiers. U was designed to perform mathematical operations with predetermined Voltage levels, Op amps were the basic butld-

Figure L Schematic symbol of a typical op-urnp.

ing blocks of analog computers, because they could perform the mathematical operations of multiplication, addition, subtraction, integration, and differentiation. The op amp used today can stilE be used to perform these operations, and now also work in other useful eir-

"The op-amp is the most commonly used integrated circuit in the industry today-"

cuit designs. In combination with nonlinear elements such as diodes, they may be used as limbers, level detectors, and nonlinear function generators. By designing op-amp circuits that include other active components, such as transistors, it's even possible to multiply and divide analog voltages by taking the logarithms and anti-logarithms of input voltages.

The modern day device tends to operate at lower voltages and does not have any of the common problems associated with vacuum tubes. Today's op amps are in integrated circuit formats and still resemble the high gain direct current amplifier which uses external feedback for controlled responses. Users working with op amps see that they adapt well to a variety of industrial applications. They can be designed to function as tillers, oscillators, pulse modulators, peak detectors, signal-function generators, small signal rectifiers, instrumentation amplifiers, and a seemingly endless variety of specialized circuit applications. The op amp is the most commonly used integrated circuit in the industry today.

The op amp schematic diagram looks like a triangle. (See Figure 1). There are two input terminals used to correctly address input signal information to the op amp. They are traditionally drawn on the left-hand side of the schematic symbol as represented in Figure 1.

The input terminals are connected internally to a differential amplifier located inside the integrated circuit paekagc. These terminals are the inverting and noninverting inputs and carry the symbols — and + respectively. The device also has one output terminal, located at the apex of the triangle, on the right-hand side of the schematic symbol.

Under normal operating conditions, an AC signal applied to the inverting terminal (-) with reference to ground is 180 degrees out of phase with the output signal, This inversion may be difficult to see with a single trace scope- A dual-trace oscilloscope, however, compares the input with the output signal, making the phase shift easy to see. An AC signal applied to the noninverting terminal (H-) with reference to ground is in the same phase as the output signal.

Of course, the op amp needs supply voltages and external components added to operate correctly,

Circuit Configurations

Circuits using op amps commonly display properties radically different from those of the individual devices themselves. For example, the circuit's closed loop gain, AC], is only a fraction of the device s internal open loop gain. A0|. In addition, the circuit input impedance is often much different than the operational amplifier s internal input impedance (although for some circuits it will be the same magnitude). Output impedance of the op-amp circuit is usually less than the op-amp device, but the bandwidth is usually greater in op-amp circuits. It s best then to make the first analysis of this circuit assuming an ideal op-amp situation, and then to modify the analysis for the imperfections in the real world of circuit design.

The ideal op-amp has the following five characteristics: infinite open loop gain A0] — infinite Infinite input impedance Z1N = infinite Zero output impedance Z0 = 0 Zero offset voltage V03 = 0 Zero bias current lrB = 0

The previous assumptions arc by no means complete. They are, however, the values which influcncc most other characteristics. The approximations simplify the analysis of operational circuitry. For example, the assumption of an infinite input impedance til-lows the experimenter to ignore the loss of any signal current into the amplifier's input terminal. The lack of bias current enables him to neglect the effect of this variable. Designers can apply these assumptions to many circuits.

Video Amplifiers

The video amplifiers characteristics are similar to those of the garden variety op-amp. Typical performance differences between the op amp and video amplifier are superior bandwidth and gain characteristics. Whereas both devices share ''idcaf* characteristics of infinite gam, zero output impedance, infinite input impedance, and an offset of zero, the video amplifier averages a typical bandwidth between 50 and 90 MHz. This compares very favorably with the bandwidth average of 100 kHz for typical op amps. Some video amplifiers have band widths as high as 100 MHz,

The gain of a video amplifier is usually adjustable between 0 and 400, which translates to 50 dB. This compares to 100 dB for op-amps. The internal phase shift of the video

"Some video amplifiers have bandwidths as high as 100 MHz."

amplifier doesn't allow the use of negative feedback to control gain Therefore, most video amplifiers display a limited output voltage swing. For high-frequency operation, the output voltage swing is typically limited to just a few volts.

input stages of common video amplifiers are designed so that, with the addition of a few external reactive components between the gain select terminals, the gain is easily varied. The video amplifier IC can be made to function as a high-pass, low-pass, or bandpass illtcr by altering the external reactive elements between the gain select terminals, l! a potentiometer is placed between these terminals, the gain can easily be adjusted.

This project's differential video amplifier is \he SK9017. Another integrated circuit is the NB592, manufactured by Texas instruments. The SK9017 and the NE592 are similar video amplifiers that provide selectable amplification. Most video amplifiers have selectable gains of 10, 100, or400. in addition, most video amplifiers have adjustable passbands, Study the manufacturer's specifications when planning to substitute another integrated circuit for the specified one. Specification sheets are readily available from the manufacturer or component dealer from whom the builder bought his op amp.

When reviewing the manufacturer's specification sheet, the designer sees thatthe input stage of the video amplifier consists of a basic emitter-coupled differential transistor pair connected to a constant-current-source transistor configuration. This arrangement is typical of video amplifiers available today.

Oscilloscope Preamplifier

A circuit containing a single SK9017 video amplifier, which is the only active component* can be used to increase the sensitivity of an older oscilloscope or frequency counter. The circuit in Figure 2 shows a circuit which provides about 20 dB voltage gain with a frequency range from 0,5 to 50 MHz.

fhe builder can extend the low-frequency response of this circuit by increasing the value of the 0,05 nF capacitor (or try removing the capacitor). This capacitor connects in series with the input terminal. Another advantage of this circuit is that it delivers a particularly small level of input noise, measured at approximately 20 (JV over a bandwidth range of 15 MHz.

The gain is calibrated by adjusting the gain potentiometer connected between pins 3 and 10, as shown in Figure 2. One can adjust the 1 kQ trimmer potentiometer for an exact voltage gain of 10, which helps preserve the scale factor of the oscilloscope.

Wiring Precautions

The mechanical layout of this oscilloscope preamplifier is very important. Keep all leads as short as possible. When using a PC board for this project keep all foil conductors as wide and short as practical This PC board technique helps provide a low resistance and low inductance signal path. It also minimizes stray signal feedback.

Adequate grounding is the most important wiring precaution in this construction. As with all high frequency circuits, use a ground plane and good grounding techniques. At higher frequencies, device capacitances between [C terminals reduce voltage gain in the preamplifier due to decreased capaeitive reactance from an increase in frequency.

Figure 2. Schematic diagram of the oscilloscope preamplifier using an SK90I7 integrated circuit, {Note ihat ¡he fC pins are numbered differently for the Texas Instrument's ME592.)


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  • Steffen
    What are the different symbols for op amps and their meaning?
    9 years ago

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