2. Description of Related Art Figure 1 depicts a generalized block diagram for a bipolar signal detector 10 known in the art. The detector includes a first comparator 12 that compares a sensing signal Vsen with a positive threshold voltage VH, and a second comparator 14 that compares the sensing signal Vsen with a negative threshold signal VL. The sensing signal Vsen can be derived from an associated system (not shown), for example, a sense resistor.

Gate 16 generates an output signal Vdet based on the outputs of one or both of the comparators. As shown by the polarity of the comparators, if Vsen is less than VL or greater than VH, then Vdet is HIGH (or LOW) indicating that Vsen is greater than (absolute value) the threshold voltages. One disadvantage of this topology is that a negative power supply is required for the comparators and for the negative threshold voltage VL. Also, if the sensing signal is small compared to the comparator offset, the comparator offset may generate erroneous outputs. In other words, if the difference between VH and VL is within the offset of the comparator, the circuit 10 becomes incapable of detecting a small sensed signal within this range.

SUMMARY OF THE INVENTION In one aspect, the present invention provides a level shifting circuit topology.

One exemplary topology includes a first and second level shifting circuits wherein said second level shifting circuit generating a level shifted output signal from a variable input signal, the said first level shifting circuit generating a fixed level shifted output threshold signal from a fixed input reference signal. The level shifting circuits are biased with respect to each other so that each has an output signal that is shifted with respect to the other.

Another exemplary level shifting circuit is provided that includes a first, second and third level shifting. The second level shifting circuit generates a level shifted output signal from a variable input signal. The first and third level shifting circuits generate

fixed, level shifted output threshold signals from a fixed input reference signal. The level shifting circuits are biased with respect to each other so that each has an output signal that is shifted with respect to the other.

In another aspect, the present invention provides an amplifying level shifting circuit topology. One exemplary topology includes a first and second amplifying level shifting circuits. The second amplifying level shifting circuit generates an amplified and level shifted output signal from a variable input signal. The first amplifying level shifting circuit generates a fixed, amplified and level shifted output threshold signal from a fixed input reference signal. The amplified level shifting circuits are biased with respect to each other so that each has an output signal that is shifted with respect to the other.

Another exemplary amplifying level shifting circuit topology is provided that includes comprising a first, second and third amplifying level shifting circuits. The second amplifying level shifting circuit generates an amplified and level shifted output signal from a variable input signal. The first and third amplifying level shifting circuits generate fixed, amplified and level shifted output threshold signals from a fixed input reference signal. The amplified level shifting circuits are biased with respect to each other so that each has an output signal that is shifted with respect to the other.

In a specific, exemplary circuit implementation, the present invention provides a detection circuit that includes a first and second level shifting circuits and a comparator.

The second level shifting circuit generates a level shifted output signal from a variable input signal, and the first level shifting circuit generates a fixed level shifted output threshold signal from a fixed input reference signal. The comparator receives the level shifted output signal and the fixed level shifted output signal and generates a signal indicative of the difference between the level shifted output signal and the fixed level shifted output signal.

In another specific, exemplary circuit implementation, the present invention provides a detection circuit that includes a first and second amplifying level shifting circuits and a comparator. The second amplifying level shifting circuit generates an amplified and level shifted output signal from a variable input signal, and the first amplifying level shifting circuit generating a fixed, amplified and level shifted output threshold signal from a fixed input reference signal. The comparator receives the amplified level shifted output signal and the fixed, amplified level shifted output signal

and generating a signal indicative of the difference between the amplified level shifted output signal and the fixed, amplified level shifted output signal.

It will be appreciated by those skilled in the art that although the following Detailed Description will proceed with reference being made to preferred embodiments and methods of use, the present invention is not intended to be limited to these preferred embodiments and methods of use. Rather, the present invention is of broad scope and is intended to be limited as only set forth in the accompanying claims.

Other features and advantages of the present invention will become apparent as the following Detailed Description proceeds, and upon reference to the Drawings, wherein like numerals depict like parts, and wherein: Brief Description of the Drawings Figure 1 depicts a generalized block diagram for a conventional bipolar signal detector ; Figure 2A depicts one exemplary circuit diagram of a level shifting circuit topology according to the present invention; Figure 2B depicts an exemplary individual level shifting circuit as used in the topology of Figure 2A; Figure 2C depicts a graph of the DC transfer curves of the topology of Figure 2A ; Figure 3 depicts another exemplary circuit diagram of a level shifting circuit topology according to the present invention; Figure 4A depicts an exemplary circuit diagram of a level-shifting and amplifying circuit topology according to the present invention; Figure 4B depicts an exemplary individual level-shifting and amplifier circuit as used in the topology of Figure 4A; Figure 4C depicts the DC transfer curves for the circuit of the topology of Figure 4A; and Figure 5 depicts another exemplary circuit diagram of a level-shifting and amplifying circuit topology according to the present invention.

Detailed Description of Exemplary Embodiments Level Shifting Circuit Topologies Fig. 2A depicts one exemplary circuit diagram of a level shifting circuit topology according to the present invention. The following description of this topology is directed to a level shifting the input signals when Vsen is close to power supply VSS. As opposed

to the the topology of Fig. 1, the present invention provides two or more level shifting circuits, and in the exemplary embodiment three level shifting circuits Bl, B2 and B3 are provided, as described below.

Referring briefly to Fig. 2B, which is a separate schematic for level shifter Bi- B3 in Fig. 2A. Each level shifter consists of an active load transistor Mpi (i = 1,2, 3), a resistor Ri (i = 1,2, 3) and a source follower MFi (i = 1, 2,3). The input signal Von goes into the gate of source follower MFi and output from its source, then go through the resistor Ri, form the final output signal Vo.

Using small signal model analysis, we get: Vo = [gmFi/ (gdsFi + gmFi+ gmbFi)] Vin + Ib { [ (Pi + l/ (gdsFi + gmFi+ gmbFi)]//1/gdsPi} (1) Normally, gmFi » gmbFi » gdsFi, (Ri '1 (gdsFi + gmFi + gmbFi)) « 1/gdsPi So, equation (1) can be simplified as: VVin+IbRi+Ib (l/gmFi) (1=1, 2, 3) (2) From equation (2), the level shifter has a gain approximately equal to one, i. e., the level shifter has little or no amplification function. But the level shifter does have a shifting aspect represented by Ib (Ri + 1/gmFi). Thus, as will be apparent to one skilled in the art, by selecting values of Ri and gmFi one can select a desired shifting value.

Shifting, in this case, refers to amplitude shifting. In the circuit of Figure 2A, and assuming that Vsen is a signal that is close to a supply voltage VSS, it may be desirable to shift the input signals (Vref and Vsen, more fully described below) toward VDD.

Although not a requirement of the present invention, shifting the amplitude of the input <BR> <BR> signals in this manner may be useful so that external circuitry (e. g. , the comparators 12 and 14) operates in the linear region of operation for those devices.

In the exemplary embodiment of Fig. 2A, to simplify the analysis and construction, the three level shifters BI, B2 and B3 have approximately the same dimension load transistor Mpl, Mp2 and MP3. Also, the level shifters have approximately the same dimension source follower MFI, MF2 and MF3. Of course, the term approximately is to be construed broadly, and may depend on the particular tolerance required or the particular operating environment. Additionally, it is not a requirement of the present invention to have approximately the same load dimension for the transistors, but rather, is described herein as only one exemplary construction.

Shifting the input signals is therefore a function of the resistors Rl, R2, and R3 as shown in equation (2). In this exemplary embodiment, the resistor R2 associated with the

sensed input signal Vsen may be chosen to be shifted so that it resides between two threshold signals, i. e., Rl < R2 < R3. So according to equation (2), each level shifter has a different shifting value.

As shown in Fig. 2C, because the gain is roughly equal one, the DC transfer curves are three parallel 45 degree straight line for each level shifter Bl, B2 and B3. The DC transfer curves represent the inputloutput of each of the three level shifting circuits.

Level shifter B2 will shift the signal to be detected Vsen to VO95 which may be optimized to around the middle of the power supply, e. g. , (VDD-VSS) /2. Level shifter Bl generates a lower threshold VL', and the input to Bl is a reference signal Vref. Level shifter B3 generates an upper threshold VH from the reference signal Vref. Thus, level shifters B, and B3 generate a fixed, level shifted output from a fixed input, where the output is level shifted according to equation (2).

As mentioned above, the exemplary topology of Figure 2A assumes that Vsen is close to or equal to the power supply VSS. "Close to"is to be construed broadly and may depend on, for example, the tolerances required for a given application. For example, "close to"may be defined as within 10% of a power supply voltage.

Accordingly, Vref is selected to be close to or equal to VSS.

For equation (2), the threshold I VH'Vo'|= Ib (R3-R2) (3a) I VO'VL'I = Ib (R2-Rl) (3b) Because the gain of the level shifter is roughly one, the original threshold I VH-Vref I I VH'-Vo'I Ib (R3-R2) (4a) | Vref-VL| = | VO-VL) I = Ib (R2-Rl) (4b) As we can see from Fig. 2C, when the signal to be detected Vsen is between VL and VH then the shifted signal VO) will be between VL'and VH', otherwise VOS will be greater than VH'or less than VL'. When the topology of Figure 2A is used as a detection circuit (as described with reference to Figure 1) using comparators 12 and 14 and XOR gate 16, then when Vsen is between VL and VH, Vdet will keep the same state, either high or low. But when Vsen < VL or Vsen > VH, then VOX < VL'or VO > VH'and Vdet will change state, either from high to low or from low to high.

In Fig. 2A, the bias circuit generates the bias current Ib.

Ib = Vt ln (A)/Ro (5) Where A is the area ratio of transistor Q1, and Q2. Substrate (5) into (4),

| VH-Vref | = Vt In (A) (R3-R2)/Ro (6a) I Vref-VL j-Vt In (A) (R2-Rl) l Ro (6b) From (6a) and (6b), the threshold is determined by transistor area ratio and resistor ratio and resistor differences, which can be accurately controlled in IC process.

Of course, for those skilled in the art, the bias circuit is not necessarily the same as in Fig. 2A. Other kinds of circuits can be used to generate the bias current Ib.

Also, depending on the desired application, it may not be necessary to have both the lower threshold VL and upper threshold VH. Instead, it may be desirable that only one threshold, either VL or VH is needed, in this case, one level shifter and one comparator, either level shifter B, and comparator 12 or level shifter B3 and comparator 14 can be omitted.

Fig. 3 is another exemplary level shifting topology of the present invention, and may be used, for example, when Vsen is very close to power supply VDD. For those skilled in the art, this exemplary embodiment follows the same principles as described above with reference to Figures 2A, 2B and 2C. The changes are the source followers MFIN MF3 change from PMOS to NMOS, and the active load change from PMOS Mpl - Mp3 to NMOS MNl N MN3. The operation of Figure 3 is similar to Figure 2A except that the outputs of the level shifting circuits are shifted away from VDD and towards VSS. Of course, the circuits of Figures 2A and 3 are equally applicable for bipolar <BR> <BR> signal level shifting, i. e. , where Vsen resides at +/-0 Volts. In such a case, it may be desirable to down shift (Figure 3) or up shift (Figure 2A) the outputs so that the output reference signals VH'and VL'are both positive or both negative. These and other modifications may all be obtained using the topology of Figures 2A and/or 3.

As previously stated, the exemplary embodiments of Fig. 2A and Fig. 3 may be adapted to shift the signal to be detected, Vsen, to be around the middle of the power supply. If the topologies of Figure 2A and/or Figure 3 are adapted to include comparators 12 and 14, by shifting to around the middle of the power supply region allows the comparators to work in their linear region.

But if the signal to be detected, Vsen is small, e. g. the threshold | VH ~ Vsen | or Vsen-VL I is comparable to the offset of comparator 12 and/or 14, then it may be desirable to amplify the level shifted signals. Amplified level shifting circuits are described below.

Amplified Level Shifting Circuit Topologies

Fig. 4A is an exemplary amplified level shifting circuit topology. Like previous exemplary embodiments, the circuit of Figure 4A assumes Vsen is close to or equal to VSS. In this exemplary embodiment, three pre-amplifier stages Al A2 and A 3 are used to both amplify and level shift input signals, as will be detailed below.

Fig. 4B is the schematic for pre-amplifier Al ~ A3 in Fig. 4A. Each pre-amplifier consists of two branches. MpAi, MNAi, and Ri (1 = 1,2, 3) form the left branch, MPBj, MNBi and Rb form the right branch. The left branch shifts the input signal Vin, which is close to VSS, to an appropriate level Vg, Vg = Vjn + Ib Ri + VgsNAi (7) Where (IbR + VgsNAi) is the shifted factor. In this exemplary embodiment, the right branch is a common source amplifier. Through small signal AC analysis, we get: Vo = {gmNBi/ [RbgdsPBi (gmNBi + gmbNBi + gdsNBi)- (gdsNBi + gdsPBi)]} Vg ~ {gmNBi/ [RbgmNBigdsPBi- (gdsNBi + gdsPBi)]} Vg (8) ~ ß Vg (9) From (8), we see that the right branch has a gain P. Thus, one skilled in the art will recognize that by selecting an appropriate resistor Rb value, the pre-amplifier will work in the linear region having a gain (3. Thus, a small difference in input signal Vg, which is the shifted signal of Vin, will generate a larger difference in output Vo.

Referring again to Fig. 4A, the three pre-amplifiers Al, A2, and A3 have approximately the same dimension for all the relative transistors and approximatly the same value for Rb, but, in order to shift the input signals a different value is selected for each of RI, R2 and R3. To shift the signal to be detected Vsen to a point between two threshold signals, then the values are selected as Rl < R2 <R3. According to equation (7) and (8), each preamplifier circuit shifts the input signal and thus generates a different transfer curve, as shown in Fig. 4C.

Pre-amplifier A2 shifts and amplifies the signal to be detected Vsen to a desired output level Vo". As before, this output value may be selected to be around the middle of the power supply. Pre-amplifier Al generates an upper threshold VH"from a fixed reference signal Vref. Pre-amplifier A3 is used to generate the lower threshold VL"from the reference signal Vref. In the example of Fig. 4A, Vref is selected to be close to or equal to VSS.

According to equation (7), the original threshold

| VH ~Vref | = Ib (R3-R2) (lOa)<BR> | Vref-VL = Ib (R2-Rl) (lOb) But after the pre-amplifier, the original threshold will be amplified to <BR> <BR> | VO"-VL"|= ßIb (R3-R2) (11a)<BR> I VH"-Vo"I = (3 Ib (R2-Rl) (l lb) As we can see from Fig. 4C, when the signal to be detected Vsen is between VL and VH then the shifted and amplified signal Vo"will be between VL"and VH", otherwise Vo"will be greater than VH"or less than VL". When the topology of Figure 4A is used as a detection circuit (as described with reference to Figure 1) using comparators 12 and 14 and XOR gate 16, then when Vsen is between VL and VH, Vdet will keep the same state, either high or low. But when Vsen < VL or Vsen > VH, then Vo"< VL"or Vo"> VH"and Vdet will change state, either from high to low or from low to high. So, even if the original threshold is only about 1 mV, which is comparable to the comparator offset, the gain can be adjusted such that the output signals are well larger than the comparator offset. For example, ß may be selected to be equal to about 100, then the amplified threshold will be 100mV, which is typically much larger than the offset of a typical comparator.

Again, the bias circuit in Fig. 4A will generate the bias current.

Ib = Vt ln (A)/Ro Then, the original threshold in (10a) and (10b) will be I VH-Vref I = Vt In (A) (R3-R2)/Ro (12a) Vref-VL = Vt In (A) (R2-Rl)/Ro (12b) The threshold is determined by transistor area ratio and resistor ratio and resistor differences, which can be precisely controlled in IC Process. Of course, those skilled in the art will recognize that alternative bias circuits may be used to generate the bias current Ib. Also, depending on the desired application, it may not be necessary to have both the lower and upper threshold VL and VH. In this case, one pre-amplifier and/or one comparator, either Al and comparator 12 or A3 and comparator 14 can be omitted.

Fig. 5 is another exemplary amplified level shifting topology of the present invention, and may be used, for example, when Vsen is very close to power supply VDD.

For those skilled in the art, this exemplary embodiment follows the same principles as described above with reference to Figures 4A, 4B and 4C. The operation of Figure 5 is

similar to Figure 4A except that the input signals are shifted away from VDD and towards VSS. Of course, the circuits of Figures 4A and 5 are equally applicable for bipolar signal level shifting, i. e., where Vsen resides at +/-0 Volts. In such a case, it may be desirable to shift down (Figure 5) or shift up (Figure 4A) the outputs so that the output reference signals VH"and VL"are both positive or both negative. These and other modifications may all be obtained using the topology of Figures 4A and/or 5.

Note that if the input signal is a small signal, the topologies of Figure 4A and 5 may be used as a detection circuit for"small signal"detection. Small signal may mean, for example, that the signal to be detected is within the tolerance range of given components. Note further that the amplifying function of this circuit topology may be chosen to make the output threshold to be larger than the offset voltage of a comparator (12 or 14). Those skilled in the art will recognize numerous modifications to the exemplary topologies of the present invention. For example, those skilled in the art will recognize that many different level shifting circuits exist which may be modified to be biased with respect to one another so that the outputs are shifted in a manner according to the present invention. Indeed, the specific circuit topologies disclosed herein are only exemplary, and other level shifting topologies may be used instead. Likewise, the exemplary level shifting and amplifying circuit topologies described herein may be replaced with other level shifting and amplifying circuit topologies as are known in the art, without departing from the present invention. All such modifications are deemed within the scope of the present invention, only as limited by the appended claims.