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Designing with a New Super Fast Dual Norton Amplifier

National Semiconductor Application Note 278 Timothy T Regan September 1981

Designing with a New Super Fast Dual Norton Amplifier
WHY ANOTHER NORTON AMPLIFIER The current differencing Norton amplifier has been widely applied over the last 5 years because of the versatility and availability of quad Norton amplifiers (the LM3900) These low cost quads are found today in a wide variety of analog systems but primarily in medium frequency and single supply AC applications Today a brand new dual current differencing amplifier the LM359 offers spectacular speed improvements which can be used in circuits operating well beyond the video frequencies How the speed is improved The speed improvement of the new Norton amplifier is due to the cascode circuit (Figure 1 ) Cascode circuits are used in high frequency singleended amplifier designs because there is no Miller effect on the collector-to-base capacitance of the input transistor Also there is no collector-to-emitter parasitic feedback in the common base configured transistor Q2 so the high frequency signal appearing at the output of the cascode does not reflect back into the input Furthermore note that bandlimiting PNP transistors are eliminated from the signal path here PNPs are used only for collector loads so not only is high speed maintained but high gain is also obtained without additional amplification stages
National Semiconductor Application Note 278 Timothy T Regan September 1981
A NEW HIGH FREQUENCY ACTIVE FILTER STRUCTURE Multiple op amp active filter building blocks are very popular because of their low sensitivities and their tunability The basic element of such a filter is the inverting integrator Usually two inverting integrators are cascaded and a third inverter allows closing the overall loop with the proper phase This is the idea behind the state variable and bi-quad filter structures which today are fully available in low cost hybrid forms
FIGURE 2 Adding a Current Mirror to Provide Current Differencing Inputs
FIGURE 1 Basic Cascode Circuit Adding a mirror to get differential inputs To make the high frequency single-ended amplifier more versatile differential inputs should be provided An easy way is to add a current mirror across the negative (inverting) input terminal (Figure 2 ) This method provides current differencing as the current entering the non-inverting input is extracted from the inverting input current The LM359 is then a current differencing as opposed to a voltage differencing op amp The programmable features extend versatility An additional feature of the LM359 is the programmability of its speed its input impedance and its output current sinking capability for line driver applications and for control of overall power consumption (Figure 3 ) An internal compensation capacitor is adequate compensation for all inverting applications where the gain is 10 or higher An additional compensation capacitor can be added externally to reduce undesired bandwidth or to fit any particular application as will be discussed later The following sections illustrate some new design ideas using this fast Norton amplifier
FIGURE 3 A Simplified Schematic of the LM359 a High Speed Current Differencing Amplifier The Input Output and Speed Characteristics are Externally Programmable
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C1995 National Semiconductor Corporation
TL H 7490
RRD-B30M115 Printed in U S A
The op amp count in these filters could be reduced by one (allowing use of a dual op amp instead of 3 op amps or a quad) if a true non-inverting integrator could be built with a single op amp Unfortunately this cannot be done with standard op amps but is a trivial task with current differencing amplifiers (Figure 4 ) Combining a non-inverting integrator with an inverting one a new high frequency and low sensitivity active filter building block can be made (Figure 5 ) Table I shows the 3 particular filter structures together with
their design equations which are derived from Figure 5 The frequency compensation for the 2 amplifiers is asymmetric to optimize performance Also since the LM359 is a wide bandwidth amplifier high frequency circuit layout is strongly recommended The circuit works with a single supply and the output DC biasing of each filter type is provided with 2 resistors R1 and Rb which should be chosen according to Table II
TABLE II DC Biasing Equations for VO1 (DC) j VO2 (DC) j V a 2 Type I Type II Type III 2 VIN (DC) 1 1 2 a a e R1 e 2R V a (Ri2) R RQ Rb 1 1 2 a e R1 e 2R R RQ Rb 1 1 2 1 V (DC) 1 a e e IN a R RQ Rb R1 V a (R i1) 2R
FIGURE 4 A True Non-Inverting Integrator
Table I and Table II relate to Figure 5
FIGURE 5 High Performance 2 Amplifier Bi-Quad Filter Half of the LM359 Acts as a Non-Inverting Integrator and the Other Half Acts as an Inverting One No Extra Inversion is Necessary to Provide Proper Phase
The simplest method to dynamically control fc is to vary ISET IN through a control voltage VC where ISET IN e VC b VBE RSET IN a 500X
In this manner CCOMP should be chosen for the highest desired corner frequency at maximum ISET IN Two curves illustrating the dependence of the corner frequency on ISET IN for two different compensation capacitors are shown in Figure 7
FIGURE 7 Amplifier Closed Loop Corner Frequency vs ISET IN It should be noted that as the compensation capacitor is increased or ISET IN is decreased the maximum slew rate of the amplifier is decreased To prevent slew rate induced distortion of sinusoidal input signals the following restriction applies Slew rate max e 3 ISET IN t 0 Vo peak CCOMP
where Vo peak is the peak output voltage of the filter and 0 is 2 q fIN where fIN is the signal frequency The output voltage for signal frequencies less than the corner frequency of the filter (within the passband) should then be restricted to Vo peak s VT b
FIGURE 6 Voltage-Controlled Low Pass Filter Minimum Input Frequency is Determined by C1 and R1 The closed loop corner frequency which as stated is also the corner frequency of the filter is fc e b GBW e b AVOL fp where b is the feedback factor R1 (R1 a R2) and a single pole open loop frequency response is assumed Combining these two expressions the corner frequency is fc e 3 ISET IN b 2 q CCOMP VT
VIDEO AMPLIFIERS The basic principle behind the design of the LM359 is to provide amplification of high frequency signals with the ease of using standard operational amplifiers The most obvious application area for this amplifier is in the video area where a fair amount of gain is required at frequencies much higher than monolithic op amps can provide A specific application is the amplification or buffering of a composite video signal for a distributed monitor system
Figure 8 shows a typical connection for a non-inverting video amplifier whose signal source may be either detected video from a receiver or possibly a camera signal The output stage of the LM359 can be programmed as shown to drive a terminated 75X cable to 4 Vp-p for use as a video line driver For color signals the differential phase error and differential gain error at 3 58 MHz are desirably low as noted in Table III
pulses that can be processed by data separating or decoding circuitry The two amplifiers in a single LM359 package can be combined in a variety of ways to provide the basic blocks of a playback channel a) For very high bit rates and low level signals they can be cascaded to optimize overall gain bandwidth product as already shown in Figure 9 b) For single-ended playback signals (non center-tapped head) one amplifier can be used as a gain stage and the other as a differentiating stage to convert recovered signal peaks into bi-directional zero crossing signals and then properly drive a comparator with regard to direction of flux changes on the disc or tape this simplifies decoding of phase-encoded data c) For differential playback signals (center-tapped head) one amplifier can be used to provide gain for each output signal individually to retain the differential signal or a single amplifier difference amp can perform a differential to single-ended conversion and the other amplifier can perform differentiation of the single-ended signal For multichannel parallel recorded data the overall component count of the playback system can be minimized by using one amplifier of the LM359 per channel Combining gain with constant delay filtering Another important application of the LM359 in data recovery systems is that of filtering It is most desirable to prevent high frequency noise spikes from being coupled through the sensing stage causing erroneous readings but the low
For general purpose wideband amplifiers the availability of two amplifiers in a single package allows cascading two gain stages to achieve very high gain bandwidth products as shown in Figure 9 DISC AND MAGNETIC TAPE MEMORY SENSING In digital data recovery from a magnetic storage medium such as a disc or magnetic tape there exists a need for high gain bandwidth amplifiers to convert the low level voltage transients from the output of the playback head (caused by a magnetic flux reversal on the tape or disc) to digital
FIGURE 8 A Typical Application of this Fast Norton Amplifier as a High Perfomance Video Amplifier Driving a 75X Line
eOUT j 1000 eIN Circuit BW j 8 MHz
FIGURE 9 General Purpose High Gain Wideband Amplifiers Can Be Obtained by Cascading the 2 Norton Amplifiers Available on a Single Chip
eOUT e 100 eIN fO e 250 kHz Time delay e 636 ns for f s 250 kHz
FIGURE 10 A Fourth Order 250 kHz Bessel Filter for Data Recovery Systems The Filtering Function is Done with a Single Package
FIGURE 11 nVBE Biasing Can Reduce Input Noise Voltage
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b) Noise Performance of Figure 11
VOS null (VOS is typically k 25 mV) IBIAS k 50 pA
FIGURE 13 Combining the Norton Amplifier with Discrete P-Channel JFETs to Make a Fast Voltage Mode Op Amp
FIGURE 14 A High Input Common-Mode Voltage Difference Amplifier
FIGURE 15 Using a Fast PLL to Make a High Frequency Ultra Linear V F
x 2N5038
Full-scale adjust made with VIN e b 10V Zero adjust made with VIN e b 0 1V
FIGURE 16 Complete Schematic of an Ultra Linear Two Decade (50 kHz x 5 MHz) VCO
Designing with a New Super Fast Dual Norton Amplifier
FIGURE 17 Typical Performance
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